Do you mean RCM and the Revenue Cycle Model and Lead Lifecycle?
Marketo usually suggests editing the model, but if the changes are dramatic, I'd say build a new draft model, then plan out the new RCM and LLC campaigns in a separate program. In fact, I'd do a draft new model and then decide if I could edit the original RCM and values to retain the historical data.
But if you do either option, some of your data will be cleared out and your reports will essentially start over. You will also need to rebuild some reports if you completely rebuild the system.
Edit a model: https://community.marketo.com/MarketoTutorial?id=kA250000000KzWoCAK&refURL=https%3A%2F%2Fcommunity.marketo.com%2FMarketoDeepDive%3Fid%3DkA5500000008QOkCAM
I am not an expert, but we just made a pretty major change to our model. We cloned the existing model, made the changes, then built some campaigns to level set leads to the new proper place, then turned on the new model. We archived the old one so as to 'document' last year's plan.