In normal times training a pilot means removing him/her from the roster so there is a cost there, but since we're talking about people mostly sitting on their asses right now, what is the cost exactly ? Especially for airlines owning simulators, training material etc. ?
Quite a few costs.
One, if you are going to retrain a substantial number of pilots all at once you need an substantial number of extra instructors and training staff. It takes months to qualify extra instructors and they are usually paid additional wages for the instructor qualifications.
Two, airlines only have a limited number of simulators and aren't really prepared for mass retraining so they may have to hire out simulators from third party providers at a premium.
Three, not just simulator training is conducted but also training in the actual aircraft in flight (line training). You can do this via base training (with no revenue passengers onboard) which is an additional cost.
Four, it's almost impossible to do line training at the moment because barely any flights are running.
Five, instead of being furloughed you'll have to keep all these additional pilots and instructors on a salary for months (probably years) whilst they are being trained,
Six, it's not just from one fleet to another. So for example you have an airline with 777, 787, 767 and 320 and you were just getting rid of the 777,if the contract allows for it those moves may be made downwards from the 777 to the 787, then the excess 787 pilots to 767, then excess 767 pilots to 320 etc, which will just cascade the effect down and blow out the cost exponentially.
Six, when the passenger loads pick up in a few years time you get to go through the entire process again in reverse.
Trust me, additional pilot training is a huge cost to airlines and they do their upmost to prevent it from happening. Airlines usually "freeze" their pilots to one type for several years to prevent to much retraining, but if the market picks up quicker then those training freezes will have to be ignored.
There's other negatives to a retraining process like this in terms of operational experience, knowledge, team togetherness, morale, etc but cost is the main detracting point for airlines.