Ephemeris data required
But won't that also be using the same length of week number ? At least, I'd have hoped they'd be consistent.
If they didn't do something stupid and use different formats, then the device would just think it's at some time in the past - and so are all the satellites. The time of day may be wrong (not sure why as it would only be the week that's wrong) but the position should still be correct. Might be some problems around rollover time if the GPS uses a different format internally - but that's likely to just cause it to have to do a completely cold start and find a satellite to download data from again.