Can an 11B switch over to become an 11C? If so, what are the requirements?

Having served a good portion of my Career as an 11B I can say that becoming an 11C in your comp[any would be as easy as volunteering to be reclassified, When you hit SSG they used to select 11C as a secondary MOS.

Regular Army is going to treat this differently than National Guard. My service overlapped part of 1SG Hicks. In Regular Army because each Infantry BN had 11C and both 11B and 11C are the same job family, you didn't need to attend AIT for 11C they would just flip you over to a 11C squad and you would start a course of supervised on the job training with the unit and they flipped your Primary MOS via orders at some point from 11B to 11C. They did the same 11B to 11H when there was a shortage of TOW folks. National Guard is probably different due to the geographic location of the units being more spread out and the training being less than every day. Even though I served two years in the NG........I have no clue how they do it. So when I was in all you needed to do was ask your 1SG for a transfer to a 11C squad and usually he can arrange one in the same BN. The problem your going to run into is he needs someone to fill your slot so it might be easier if you found a 11C that wanted to be an 11B about the same rank as you and in the same area you want to switch to.............thats not a requirement though as he can also work out a deal with that Company's 1SG that he gets the next new guy from Infantry OSUT.

You should be able to but interesting question nonetheless. Which means that they no longer do cross-over training during Infantry AIT. When I was going through the Infantry School in 1980 we did one week of 81mm mortar training (11C) and one week of TOW training (11H). We had guys switch between the 3 MOS' during AIT. But as a LEG Infantry Training Company the Hotels Charlies would go their way and we would go ours.