Differences training relates to the aircraft rating, not the licence, so it doesn't matter whether you are exercising PPL or CPL privileges.
VP prop privaleges on a SEP are forever and grandfather rights apply if you have flown them before, so no sign up required. If you fly a twin with VP props you must maintain curency, 24 months I think. In reality, an annual check out covers that, but you could go more than 2 years without flying a turbo twin so differences training and a signature is then needed to revalidate the turbo part. As there is no specific place in the log to put the signature the whole thing is a bit of a nonsense.