Can a 12-25 cogset be changed to 12-27 easily?

[OakLeaf]

just a cable and housing change.

YMMV, but IME there's not much "just" about replacing cable housing.

When you cut cable housing you have to (1) make sure you don't crimp the housing when you cut it, (2) use a smooth punch to open up the hole when you inevitably DO crimp the housing, and (3) file all the burrs off the cut end(s) so they don't catch and fray your cable.

A good housing cutter that will HELP cut without crimping, but not guarantee it, is like $50 (more than the cassette tool and chain whip put together), and will get very little use unless you've got a stable of bikes that all get ridden frequently. You will also want a small round file to get the inside surface, plus ideally a flat one also to get the outside and the edge most efficiently, though you can do it all with the round one - might or might not be something already in your toolbox.

Then you have to re-tape your handlebars. Ugh. I know some people here can do that all day, but I don't get enough practice, and I always wind up re-doing it six or seven times a side before I can get it smooth, and there's usually still a lump at the bar end. Actually the last time the shop did it for me, the mechanic's 11-year-old daughter did it flawlessly while I watched. SHE gets to practice on other people's bikes.

Sizing and routing the housing isn't a big deal - important for safety, but easy - but cutting it, I still say is a PITA and well worth whatever the shop charges to do it, which is usually minimal.