I installed them when I replaced the wheel bearings on my camper. Worked great, would give them a lube job at the beginning of the season and a few squirts here and there. Did a complete grease change once in the 3 years before the lift system broke.
This winter will be balls to the wall with turning it into a utility trailer.
As for purging the old grease, they use the same concept on aircraft grease points. Pump grease until the old starts to come out, then keep pumping until new grease shows up. Easy on some things, PITA on a CH-53E, we put 5 tubes of grease into the main rotor swashplate one day. That is alot of pumping.
Just keep pushing out grease until it changes consistency or color and wait until only the right grease comes out.