When moving to the subscription offering you will need to retire your perpetual license. There are some rules with regards to what software you can switch to.
If you have an individual point product like AutoCAD or AutoCAD Electrical you can move the client to either the corresponding point product or to an Industry collection.
If you have an individual LT product (Inventor LT, Revit LT and AutoCAD LT) you can take it to the corresponding point product or to an LT suite.
If you have any of the old Design suites (Product Design suite/Building Design Suite) you can move it only over to an industry collection.
If you have an LT Suite you can only move it over to an LT Suite as well.
With regards to the length of the contract. If you were on a yearly or multi year maintenance plan, when you switch you can go onto any subscription plan as well.
If you decide not to renew your maintenance plan you are more than welcome to and you will stay on the latest version that you received from Autodesk. What you will not be able to do is use any previous versions as well as not be able to make use of the extra territory rights that staying on maintenance or subscription allows you to.
If you do decide to get the latest version you will have pay for a new subscription at the current pricing not the special switching price that Autodesk is offering.