If you have the choice about the matter for phone company Internet access, provide your own DSL modem, for cable internet access, provide your own cable modem. When they apply firmware upgrades, it will only affect this piece of equipment
Then run a separate Ethernet Firewall/Router with builtin Wireless Access Point so you have reasonable control over your own home security. You can then keep this part of the hardware package upgraded to the latest firmware from the manufacturer and retain control over your firewall administration access and assigned SSIDs.
The reason for these changes you mentioned is that ISP owned DSL and Cable Modems require security and firmware stability upgrades just like everything else on the planet.
The ISP has stock setups that get pushed, and just like doing firmware upgrades to your own equipment, the current configuration settings may not survive the firmware upgrade so they have defaults that get reapplied so the darn thing at least works. Doesn't bode well if it resets admin access credentials system-wide to something that's known, at least they notify you so you might be able to change them to something more secure.
BTW, was this Canadian DSL equipment (HomeHub 1000/2000)? There were some egregious issues reported earlier this month with a hard-wired password on them and the issue you mentioned is actually a very desirable side issue to the firmware upgrade they need to protect you if they've finally got around to it. Qwest DSL modems from a certain manufacturer might also have the same issue as well.