The 71M will still work if the memory battery is dead. It is only there to store channels.
By the sounds of it they did more than just replace the memory battery.
I can do that in about 20 mins. Open the top cover, remove the old one and solder in the new one. Replace top cover. Done.

Still the best and most reliable place to have any work done is with Anton Lang in Germany. He has all the spares you can ever need.