I think the entire European map is 2.2 gigs and the World Traveller has 4 gigs internal storage.
Don't try anything without a complete Explorer backup.
I, too, have the same model and, as I remember, Home was prepared to download 2 different European zones (think the east is small) at the same time while still keeping the NA map loaded.
But you'd have to check in Home as to which countries are covered by which zones such that the both maps you require may be able to be downloaded at the same time.
So, if you DO want to try to get the whole European map:
1. Make that Explorer backup.
2. Then use Explorer to move the North American map off the unit and into a folder on the computer, leaving your original full European map installed.
Then try for the full European map update.
BUT.............
There is another issue. When 'lifetime updates' don't mean that......
Others report that they get the updated North American map ok but TT refuses to give the lifetime update to the European map installed.
HOWEVER...........
If you're trying for the update via the LMG, then possibly the method I described will work.
Something else to keep in mind............
If your current maps are not 850 and you just got the unit, be aware that 855 maps are due for release within the next week or 10 days, so you may want to wait if that's still within your 30 day window.