Financial Account cannot be change to non deductible
This is a core bug and have reported issue at https://lab.civicrm.org/dev/financial/issues/28  and also submitted fix at https://github.com/civicrm/civicrm-core/pull/12500 

Can you please try after applying the patch from here?

If the patch works without any error than please comment on the PR as it would help for PR to get merge quickly into core.

HTH

Pradeep