maybe you can upgrade the sentry ability to increase guns on it or type of ammo just a thought nothing really behind it so we're clear.
Theres already upgraded sentrys, you can get them by getting the skill, theres level 2 and 3 sentrys you can get after unlocking the initial sentry skill i belive.
Getting sentry 2 will replace the first sentry and so does sentry 3, you cannot use sentry 1 afer you get sentry 2 on that character, as with sentry 2 with 3.