Will i keep my infantry signing bonus if i switch from 11b to 11c?

11X means Infantry unassigned. Every 11X becomes an 11B or 11C which are both Infantry, so you get your bonus.
Reclass once you have an MOS involves a lot of rules based on your circumstances and the policies in place at that very moment in time.
So, the answer is yes you could, but no you probably would not.

Good luck trying to reclass from 11c. As of right now the only way to reclass is to drop a packet MOS or graduate Ranger school as lower enlisted and hope your commander will sign off on your 4187

In the press release on Military.com the bonus stated it was for enlisting 11X so the answer is it would not reflect your bonus. Also, I enlisted in the 1980's which was a long time ago so take this bit of advice with that in mind because I have no clue if this has changed or not. It was not really considered a reclass if you switched from 11C to 11X or vice versa because you remain 11 series. In fact when I was in, you could basically ask your First Sergeant and he would make it happen (I guess I should say He or She now). Here is where your going to run into pushback though. Show up to your first duty station as a 11C and they are going to want you to stay 11C because of the additional training you have for it. However, typically if your 11C and your not really happy with the MOS after say 12-18 months they possibly would let you switch. It's only a little paperwork via BN to switch your PMOS from 11B to 11C or vice versa and they do not have to send you through AIT again they can do it just by transfer to another unit in the BN and on the job training. At least that is the way it used to work. In my view you should think long and hard before switching to 11B from 11C because 11C is 11B when they have no need for mortars. So the switch does not buy you a whole lot. I can say that because long ago I was 11H (TOW Missile) and we did 11B crap 4-6 months out of the year because what happens in a Infantry BN is the 11B's start to snivel if they saw the 11H's having it too easy and the Sergant Major intervenes and let the 11H 1SG know that they need to do more 11B stuff (yeah no kidding thats how it worked). Usually the 11B's kept quiet about the 11C's because 11C's have the reputation of having it tougher because they have to carry their heavy assed mortar baseplate into the field and no 11B wants to get stuck doing that via complaining. So generally the 11B's only complained about 11H until the MOS was eliminated. There was no real rivalry between 11H and 11B though, just heard via the NCO chain they complained sometimes and that is why this week we were doing 11B tasks (so it was kind of humorous). Anyways, good luck on your future career............11 Series is kick azz in my opinion because of the schools they offer and the doors it opens if you stay Active Army. The above does not apply if your National Guard because NG you enlist for the MOS specifically then they send you to training, which if that is still the case, the above is different because NG works more on a slots available system as well as a unit manning roster much harder to switch 11C to 11B and/or back again if you can at all.