> Oh well that changes things.

It's the same for any case where someone sends you something and wants it sent back with changes.

If you screw up their files it's never going to go well for you (family, friends, volunteer groups).

Nobody will ever thank you for messing up their work because it's now saved in an open format.

If they end up using a feature that triggers the format nag screen they'll likely end up saving it back into the native Microsoft format anyway.