Note that you can add in more for add-on purchases, but I don't think you'd be able to up your "pledge category"
Yes, you can.
MERCS: Recon Kickstarter Update 37 wrote:
We are going to have a pledge manager. Backers will be able to add product afterwards and upgrade their pledge level. In addition, several KS exclusive stretch goals will be able to be added-on after the fact. So if you wanted multiples of say a CPP mini (but only get one in the Kickstarter), you could add additional in the PM. This is to help those backers that are going into the KS with additional people and each backer wants a stretch goal (of if a single person wants many of one thing).