For what technical reason would the modem be at fault? The filters are in the phone socket itself, the crystal oscillators inside the modem aren't going to have degraded to a point whereby they're too out of specification to correctly syncronise with the upper DSL frequencies with a 7 year use.
Switching between modems is a very big no no, I previously had to have a BT engineer called out because of slow speeds for that exact reason (switching between a HG612 and HH4), they had to go to the local exchange to reset the port because the speed was limited to about 2Mbps - the engineer said that when errors are detected e.g. switching modems, the equipment exchange side registers this as a fault on the line and reduces the encoding to prevent signal degradation, it's been a few years since I studied broadband networks but an example would be from allowing 64-QAM even with some errors, to forcing 16-QAM as the maximum, or whatever encodings are used. This was a different connection and from memory after the port reset speeds went to about 30Mbps or thereabouts and I discontinued using the HH entirely
