My contract with Sprint will end in August, 2013. I plan to get new phones and a new contract. As Photon with a locked bootloader is not a very popular phone on eBay, I plan to keep these phones and use them for visitors from abroad with a prepaid SIM card.
When my Motorola Photon 4G will no longer be on Sprint, will it be possible to unlock the phone to enable it to accept SIM cards in the US?
That won't do you much good. The SIM card is used only for the WiMAX service. Voice calls are placed over the CDMA network. The CDMA profile is built into the phone and does not use the information on the SIM card.
When you get new phones, just take whatever you can get for the old ones or keep them as backup devices. I will be getting a new phone in July when I'm eligible. I plan to keep the Photon as a backup, since it's the only old phone I have that I possibly could tolerate to use if I had a mishap with its eventual replacement.
I have read the documentation and articles about the Photon 4g when it was new and the information that I gleaned from them was the sim slot was for international travel by the user. The software that controls the sim slot "knows" when you are in the US/North America and will not work until you leave. CMDA is more popular in the States than anywhere else. GSM is more popular in the rest of the world.
The GSM sim card will not work in the Photon 4g in North America/US. If you can find software that can disable the sim slot protection, without bricking the phone, good luck. I haven't seen it also, there is no guarantee that it will work on another network. Have fun!