I would probably just get the whole frame and fork when the time comes. However you certainly could put a new fork on your old bike. 5 mm is no difference at all. Offset is another mater, you would want those to be somewhat close.
The main reason I would not go for just a fork is the cost of all the peripheral changes that go with it. You can also do perpetual tours with as little as about 10 pounds of gear, so you really don't even need greater capacity than your current ride allows, unless each additional day is in a direction where there is neither water or food resupply.