When adding new GTM(DNS) to the existing group, if software version is not same, what will happen?

hi, I have one question on GTM/DNS:  when adding new GTM(DNS) to the existing group, if software version is not same, what will happen? our existing GTM(DNS) group OS version is 13.x.x, and new DNS OS version is 17.x.x. If we want to add this new DNS to the sync-group, when should we do to make it work? Anyone pls advise, thanks in advance!