Will moving my 400K savings to a fixed account affect my visa extension eligibility in Thailand?

For Bangkok Bank accounts this will not work. When they print the statement showing the account activity and balance, they deduct the amount used as credit card collateral. So if the account has 400K in it and that amount is designated as credit card collateral, the statement will show a balance of 0 baht. Immigration doesn't care that the balance in your bank book is 400K. They go by the printed bank statement. So before you try to do it this way you need to check with K Bank to see if they do it the same way.