This is insanity.
Your riflesmith is sanctioning pulling a brand new barrel, chopping over 1/2" off the tenon, recutting/threading the tenon back to correct length...then having to dial it all in to recut the chamber; which in the Tikka contour I would have concern about placing the chamber that much further forward into the tapered section of the barrel. That's a LOT of setback.
Do yourself a favor. Find a smith to pull your barrel, sell it as new take-off to someone that wants the 7 mag and spend the few extra bucks on the new blank. I'd charge more to do what you're asking than to chamber and fit a new blank.
Besides... I've got to ask, WHY would you want to go from the 7 mag to the SAUM when you're already in a long action??