To give the OP a more generic answer: technically "held" reservations are simply non-ticketed PNRs. This means that they can be altered in any possible way, and of course without a fee.

However, united.com won't let you do that -- the "held" reservatrions from .bomb are simply non-ticketed PNRs that auto expire. The only actions you can take on them through the web site are delete or purchase.

Many TAs on the other hand can create non-ticketed PNRs that don't expire, and can change them anytime. Most likely a UA res agent can also change a held PNR.

Customers don't really need to understand these details though... since held reservations auto-expire you don't have to worry about them and if you want to make changes you can just create a new booking.