A Microvasive is similar to a BARD in that it does, yes have to be replaced by the doctor, but unlke a MIC-Key, it wont wear out fast.
It shouldnt have to be replaced unless you see any obvious signs, like tearing or holes, or if it leaks profusely when you open it up ( meaning the valve may be worn, or wearing out.
Brianna has had MIC-Key's. She started out with a BARD and had the first one from age 5 months till almost 3, wehn well, frankly, the cat chewed the flap off. otherwise it would have lasted a LOT longer.
Another reason for replacing would be stoma length change. As your child grow, the Fr size of the device doesnt usually change, however,, as their becomes more muscle, fat, skin, etc between the inside of the stomach and the outside of the abdomen, the length would change. ( this is "point cm" distance, as in my child has a 18fr , 2.0 cm Mcrovasive mini-button)
Brianna currently has a Entrastar button. It goes in like a BARD, but has a surface level valve like a MIC-Key.
We like it. We never would have switched from a BARD to a MIC-Key if it wasnt for the stupid resident that just yanked the BARD out to change it and ripped her gut apart as well.

Anyway, you can also research microvasive online.
www.bostonscientific.com

I also subscribe to this egroup, which is AWESOME if you deal with gtubes, jtubes, g-jtubes or TPN. it is called G-tube Digest

http://members.iinet.net.au/~scarffam/cgi-bin/fom?_recurse=1&file=120

hope this helps