We are planning for an upgrade from 7.31 to 8.41 and considering skimming as part of it.
The major reason for skimming would be upgrading the references of RBV enterprise-level ruleset from Pega-ProcessCommander:07-10-99 to Pega-ProcessCommander:08-04-99 and thus having a consistent new application version based on PRPC 8.41. As I see from the test migration the ruleset references are not upgraded automatically unlike application references. E.g. we decided to leave the old application UI-Kit:10.01 as a basis and after the test upgrade the dependency application of UI-Kit was corrected - it is PegaRULES:8 now