How many pages/blocks do you expect to be changed? updating using IContentRepository should be fast enough, while safer and more future proof 

My bad, I always thought it is possible to change Master Language for CMS content, which is turned out to be, not. One alternative is to copy the content and resave in a different master language. quite troublesome yes but much safer ... 

zone/Zone.Epi.ContentCopy: Adds an Episerver CMS Admin tool to copy content from one page to another. (github.com)

could be a good starter point

Finally Got the below script that works for all blocks and pages.

/****** Object:  StoredProcedure [dbo].[EPiCode_ChangePageAndBlockBranchMasterLanguage]
 Script Date: 2022-07-11 14:26:38 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[EPiCode_ChangePageAndBlockBranchMasterLanguage]
    @content_id    int,
    @language_branch varchar(20),
    @recursive bit,
    @switch_only bit,
    @delete_old_language_id int
AS

DECLARE @language_branch_id nchar(17);
DECLARE @language_branch_nid int;
DECLARE @prev_language_branch_nid int;
DECLARE @child_Id int;
DECLARE @Fetch int;
DECLARE @target_lang_version_exist int;

SET @language_branch_nid = (SELECT pkID FROM tblLanguageBranch WHERE (LanguageID = @language_branch))
SET @language_branch_id = (SELECT LanguageID FROM tblLanguageBranch WHERE (LanguageID = @language_branch))
SET @prev_language_branch_nid = (SELECT fkMasterLanguageBranchID FROM tblContent WHERE pkID = @content_id)
SET @target_lang_version_exist = (SELECT count(*) FROM tblContentLanguage WHERE (fkContentID = @content_id AND fkLanguageBranchID = @language_branch_nid))

-- for switch only check if the page exist in the selected language and only then switch
IF 1 = @switch_only 
  BEGIN
    print 'target_lang_version_exist' + str(@target_lang_version_exist);
    IF @target_lang_version_exist > 0
    BEGIN
    UPDATE tblContent
        SET fkMasterLanguageBranchID = @language_branch_nid
        WHERE pkID = @content_id AND fkMasterLanguageBranchID = @prev_language_branch_nid

        IF @delete_old_language_id > 0
        BEGIN
            --modify content properties which are not language specific
            update tblContentProperty set fkLanguageBranchId=@language_branch_nid where fkLanguageBranchID=@delete_old_language_id and fkContentId=@content_id and fkPropertyDefinitionID in (select pkId from tblPropertyDefinition where languagespecific=2)
            update tblWorkContentProperty set fkWorkContentId=(select [version] from tblContentLanguage where fkContentId=@content_id and fkLanguageBranchId=@language_branch_nid) where fkWorkContentId=(select [version] from tblContentLanguage where fkContentId=@content_id and fkLanguageBranchId=@delete_old_language_id) and fkPropertyDefinitionID in (select pkId from tblPropertyDefinition where languagespecific=2)

            --delete all old content
            delete from tblContentProperty where fkContentID=@content_id and fkLanguageBranchID=@delete_old_language_id
            delete from tblContentLanguage where fkContentID=@content_id and fkLanguageBranchID=@delete_old_language_id
        END
    END
    ELSE
    BEGIN
    RAISERROR 
    (N'The Selected page with ID:%d, cannot switch master branch since there is no version in the selected target language: %s.',
    11, 1, @content_id,  @language_branch); 
    END
  END
ELSE
  BEGIN
    IF @target_lang_version_exist > 0
    BEGIN
      RAISERROR 
        (N'The Selected page with ID:%d, cannot be translated since there already is a version in the selected target language: %s.',
        11, 1, @content_id,  @language_branch); 
      END
    ELSE
      BEGIN
        UPDATE tblContent
           SET --fkLanguageID = @language_branch_id,
              fkMasterLanguageBranchID = @language_branch_nid
         WHERE pkID = @content_id

        UPDATE tblContentProperty
            SET fkLanguageBranchID = @language_branch_nid
         WHERE fkContentID = @content_id AND fkLanguageBranchID = @prev_language_branch_nid

        UPDATE tblContentLanguage
           SET fkLanguageBranchID = @language_branch_nid
              --,fkLanguageID = @language_branch_id
        WHERE fkContentID = @content_id AND fkLanguageBranchID = @prev_language_branch_nid

        UPDATE tblWorkContent
           SET fkLanguageBranchID = @language_branch_nid
              --,fkLanguageID = @language_branch_id
        WHERE fkContentID = @content_id AND fkLanguageBranchID = @prev_language_branch_nid
      END
  END

-- get all 'for this page' or 'for this block' blocks and call itself recursively
DECLARE children_block_switch_cursor CURSOR LOCAL FOR
    select pkID from tblContent where fkParentId IN (SELECT pkId FROM tblContent WHERE contentOwnerId = 
    (SELECT ContentGUID FROM tblContent WHERE pkId=@content_id))

OPEN children_block_switch_cursor

FETCH NEXT FROM children_block_switch_cursor INTO @child_Id
SET @Fetch=@@FETCH_STATUS

WHILE @Fetch = 0
BEGIN
    print @child_id
    print @language_branch_id
    exec [dbo].[EPiCode_ChangePageAndBlockBranchMasterLanguage] @child_id, @language_branch_id, @recursive, @switch_only, @delete_old_language_id
    FETCH NEXT FROM children_block_switch_cursor INTO @child_Id
    SET @Fetch=@@FETCH_STATUS
END

CLOSE children_block_switch_cursor
DEALLOCATE children_block_switch_cursor

--call recursively for all child pages
IF 1 = @recursive 
BEGIN
    DECLARE children_cursor CURSOR LOCAL FOR
        select pkID from tblContent where fkParentID = @content_id

    OPEN children_cursor

    FETCH NEXT FROM children_cursor INTO @child_Id
    SET @Fetch=@@FETCH_STATUS

    WHILE @Fetch = 0
    BEGIN
        print @child_id
        print @language_branch_id
        exec [dbo].[EPiCode_ChangePageAndBlockBranchMasterLanguage] @child_id, @language_branch_id, @recursive, @switch_only, @delete_old_language_id
        FETCH NEXT FROM children_cursor INTO @child_Id
        SET @Fetch=@@FETCH_STATUS
    END

    CLOSE children_cursor
    DEALLOCATE children_cursor
END