If I transfer my funds from one bank to another, from Bangkok to Chonburi, will it cause problems when applying for my retirement extension?

If you were within the 2 months needed to do the application it would certainly be a problem.

After that it would not be a problem as long as the funds did not go away longer than the time it takes to do the transfer.

If you mean two diff banks it would mean having to show paper and letters from two banks  a complicated process for both you  and immigration officials, who are not fond of reading bank statements.

2 hours ago, Gandtee said:

will it cause problems when applying for my retirement extension?

Probably with immigration if it is within applying time periods, if I needed to do that I would go see immigration first.