Replacing the chassis of a Woodstock series HP calculator

[HpCalcFan]

How difficult would it be to replace the chassis (the external shell, not including the faceplate or the battery pack) of a Woodstock series HP calculator?

For example, if a HP 29C has a chassis in poor cosmetic condition, how difficult/involved would it be to replace its chassis with one from a HP 21, say? (i.e. the innards, faceplate and battery pack of the HP 29C would thus reside in the chassis from the HP 21)

In asking the above question, I'm assuming that the chassis of any one model within the Woodstock series is identical (in terms of structure and dimensions) to the chassis of any other model in the Woodstock series ... Is this assumption valid?