Yes, you can do it now - and it'll then effectively be 'in phase' with your SEP Class Rating re-validation date. So, when you need to re-validate your SEP Class Rating again in 2006, you can use the IMC Rating re-validation in May 2006 for the 'training flight' requirement. But make sure that it lasts at least an hour! Alternatively, if for some reason you don't amass sufficient flight time to re-validate by experience, ask the FE to do a combined IMC Rating re-validation and SEP Class Rating re-validation flight. If he/she's a good egg, he/she might agree to a reduced fee for doing both together - I certainly do!