I find heads don't get much beyond 300,000 mdots, and six years is good going.
If you're going to replace right head, be prepared for the left head to follow shortly after. Head replacement is quick and easy, though heads (right pondian) are £275 per head, so not entirely painless
I believe you can't tilt the printer beyond 15degrees with ink in the system. If you do need to do a full move (level2 or 3?), you'll need to drain the subtanks, which'll take a fair amount of the available ink away.
If you've got full ink tanks, that's a lot of printing available to you. If the printer was running well before the replacement warning, then I'd be very tempted to replace the head (s) and keep running it. Cheaper than buying a 900 with its pocket size carts, the canon measures ink in bottles (which are priced on a par with vintage champagne...)