I had the same worry and decided that the workaround was to leave the existing MX record in place and just give the EOP MX record priority:
MX 10 EOP server
MX 20 existing Exchange server
MX 30 ISP mail queue
If it doesn't work, then change the MX 10 record to MX 100 or delete it entirely and you are back to your current config.
Hope this makes sense to you. If not please ask questions and I'll try to answer them.
F. X.