Also, I think the important point is: let's just not say if your 'grace period' is too short, what if you see 5% of nodes hanging out there and have not upgraded? 15%, 25%? What if they still haven't upgraded after the fork because, well their admins could just go on vacation or something? What would happen to all the SPV clients connecting to them? Are you going to back down in this hypothetical situation?
If they won't upgrade, the new clients will not even sync with the old relays, because blocks will be seen as invalid by the clients.
If we are talking about SPV clients, which don't see any blocks, then their money will also not be lost. They will regain the control of their money once all of them (relay/processor + client) upgrade.
The money will not be lost in (almost ?) any case.This is a problem for soft forks as well. For soft forks we know the miners have upgraded, but the user of an SPV client have no idea if the full node he connect to is upgraded or not. If it isn't, a transaction which is invalid according to the new rules may be presented to the SPV wallet as valid (but unconfirmed).