If it is just the refoam of the mid-range units, then it is a straightforward job that you could do yourself.  I'd rate it as "easy" as there's no soldering involved just the need to be methodical and to label the wires and connections as you go.  There is no need to "shim" the centre cones and methylated spirits will remove the old adhesive when used with a No. 17 knife blade. It is helpful is you can connect each speaker to a source just to check the tone is OK as it checks the surround and cone are centred. You can purchase foam surrounds and I would recommend the glue as well from various companies.  I have replaced 2 sets in Beovox and Beolab Pentas with surrounds from the Netherlands who produce very good fitting surrounds.  Don't forget to order the backing rings that go between the back of the speaker and the surround into which they fit. If you require more work, say, crossovers then that does require soldering skills and is more difficult.  There is plenty of information on the archived forums. There is/was a service on eBay offered as well - you buy the service and send your panels of 4 speakers in for refurbishment.    

Thank you for taking the time, to respond to my request in such detail.