If it has disc brakes, you should be able to do the swap easily enough. I can't speak for the montagues specifically, but generally 26" wheeled mountain bikes can be fitted with tires of at least 700x32, and sometimes up to 700x42 or even 45 depending on the positioning of the seatstay and chainstay bridges.
If you don't have disc brakes, you have a problem. The brake posts are positioned for use with 26" wheels. 700c wheels have 31.5mm greater radius, and most V brakes can't be adjusted that far up -- and even if they could, they wouldn't work properly because the leverage ratio would be wrong for use with normal mountain bike levers.
In front, the brake issue can be solved by switching to a disc brake and a disc-compatible fork and front wheel, if the ones you have aren't already disc-compatible. In back, it's trickier (at least if your frame isn't disc compatible ... not sure, but I don't think
yours is). I think there may be some "brake extenders" that reposition your brake posts up higher, but from what I've heard they tend to be extremely limited in terms of clearing decent sized tires.
There is one other way, and it's what I did when I converted a 26" bike to 700c. I got a special V-brake from Paul Components called the MotoBMX
. At $140-ish it is insanely expensive, but FWIW it is probably the best rim brake on the market. More importantly for people like you and me, it is capable of positioning its brake pads high enough to hit a 700c rim, when mounted on a bike intended for 26" wheels. Now for your brake lever to work properly, you'll still have to switch from one intended for V-brakes (high cable pull, low leverage) to one intended for road or cantilever brakes (low cable pull, high leverage). In my case that was easy, since I was converting it to drop handlebars anyway. But still easy if you have flat/mountain bike handlebars, because for not much money you can also get levers for those that are designed for cantilever brakes.