Easy for experts but unadvisable for safety reasons if you have to ask. The safety requirements are in the datasheets and application notes at the original cell manufacturer's websites and they are all fairy uniform across different Lithium Ion suppliers. The RC folks work with building their own packs and live to tell about it, but they also post pictures now and then of batteries that exploded and burned. You first have to develop skill working with live NiMH recelling with other laptops until you are CERTAIN you can work with live batteries without shorting. The lithiums need to be soldered while they have some charge in them. Attempting to fully drain them to zero voltage even just once for installation purposes dooms them to a premature death. The internal PTCs inside the Lithiums offer some degree of safety, but best to work out how not to short elsewhere. Then you need to verify preferably by a measurement that your soldering technique does not overheat the base of the solder tabs. Usually not a problem unless the tabs are really short from salvaged batteries, and your technique is slow. Staying below the boiling point of water at the cell surface (thermal runaway begins at approximately 100 C) would be good to avoid explosion. Cost can be free, from batteries on hand, up to the prices on batteryspace.com. If you need the gas gauge to work the replacement batteries need to be somewhat matched in voltage, capacity and internal resistance, however batteries salvaged from a single used working battery often meet this criteria, and new batteries purchased as a set probably will easily meet this. If the replacement batteries have greater capacity than the originals you probably will not realize this additional capacity unless you reprogram the EEPROM capacity constant in the battery manager board. To do this you first have to reverse engineer the battery manager chip identity and the safety chip identity and look at their spec sheets (which with some math will also tell you how well the cells need to be matched), but lacking this you can just use the replacement batteries at the original capacity limit. This means generally that the least expensive, lower capacity new battery cell choice is the one to buy unless you are up to reprogramming.

People can and do recell Lithium Ion packs of many different makes and have so described on their websites, so if you can stay in the safe zone it is doable, and if you exceed the safe zone the heat of soldering alone could cause an explosion and fire that you cannot stop. If you do not yet know your limitations for sure do not attempt it. Some things like NiMH are fun experiments but for Lithium you need to be sure of your technique.