Select all books you want to affect, hit Edit Metadata. Now, in Bulk Edit Metadata, use the third tab (Search and Replace) to search the tags column and replace into the #custom_mytags column.
You will need the following settings:
Search Mode: Regular Expression
Search Field: tags
Search for: (.*)
Replace with: \1
Destination Field: #custom_mytags
This will copy the contents of the tags column for all books, into the new column. Once they are cleaned, you can simply repeat the same S&R, but switch the Search and Destination fields around, to copy in the other direction.
Do remember though -- when doing bulk changes,
be careful not to jump the gun! There is NO going back, unless you have backed up your library!
You can seriously mess up your library and delete things permanently if you are not careful.
/end boilerplate warning