We have two SERO-P accounts with staggered upgrade dates and I've never had any issues upgrading either over the phone or through Sprint.com every year. 3G, WiMax, LTE, and TriBand devices haven't been a problem. Yes, you can't use a third party store like Amazon, but that's a small price to pay. The website says that you will need to change your plan to activate the new phone, but it really just means that you have to call in or use chat to do the activation (it will tell you that when you attempt to activate online).
Right now, I can use my SERO-P upgrade to get a HTC One M8, Galaxy S5, LG G3, Nexus 5, etc. Try calling telesales at 888-882-4030 if the website isn't working.