If you haven't collected money from applicants or made any agreements with them, then they aren't harmed by you changing rent, in a legally actionable way at least. As comments point out, some people might view their wasted time as an incredible offense and seek to harm you. If you have collected application fees then you either shouldn't raise the rent or you should refund all application fees if the applicants are not interested at the higher price. Legally I don't see a problem assuming no harm is done to the applicants, but go ask a lawyer in your region to confirm.
Whether or not you should depends primarily on how much below market you actually went. If you listed 20% below market value, then people wouldn't be shocked if you indicated it was a listing mistake. If it's 5% under market value, you come across as someone who is trying to squeeze as much out as you can. Some grey between those ranges, but don't expect current applicants to stick around at a higher rate.
I consistently rent on the low-end of market value because I want a wide tenant-pool to choose from. High quality tenants that feel they are getting a good value are worth a lot more than getting the highest possible rent. You want a fair rent that you're comfortable not raising too significantly if your tenants are interested in renewing. That's my model at least, I'm happy to sacrifice some rent to keep turnover low and maintain high quality tenants.