Like @eclecticsynergy said above, you'll need to replace pick guard to accommodate the neck p/up, and change the bridge plate for that p/up. If I loved the neck and the body wasn't too heavy, I would do that. If not, it's a lot of work for a guitar you might not enjoy. (get the right neck and body, and anything else can be changed)I'm looking at buying a tele with humbuckers in it and changing the pickups to single coils. Is there any problem with putting singles could in a tele body that's routed for humbuckers?
More than the bridge plate, you'll have to route out the bridge pickup area to accommodate the size and shape of the humbucker. This is why for the bridge position you'd be best off going with a single sized humbucker like the Seymour Duncan lil'59 or something like that.
Don't be in a hurry to get rid of the mini hb in the neck, if you find it a bit wooly a firebird type pickup should fit right in. I have one in a Squier and ho boy..Here it is. They go for $600.00 new. I can get it for $400.00. I can mod it if I need to and hopefully have a nice tele.
Humbucker sized p 90?It's just a really cool looking min fender tele, that I can buy cheap enough to mod. Its a Michael Kelly 1955 with a quilted maple top. It has a mini humbucker in the neck position and and full size humbucker in the bridge. Don't know much about the brand, but supposedly the fit and finish is good. Just not sure I'm gonna like the pickups. I'm a single coil guy.