Hi AirJordanFan93,
You typically have 18 months from when an area is considered ready for service by NBN before any switch off will occur. You should be able to sign up to one of the bundles and move it across to NBN but in most cases Telstra will simply cancel your old contract and give you a new NBN one as long as you're sticking around. Basically Telstra will hand over the network to NBN and NBN will install their own modem into the HFC socket and Telstra should then be able to supply you with a Gateway which connects to all your household devices.
The plans are all the same regardless of technology type and are actually set by NBN themselves so you should be able to choose from 25/5, 50/20 or 100/40 speeds. HFC and FTTP should give you speeds fairly close to these plans at times but I doubt most people would ever see 100/40 on FTTN unfortunately. So depending on the plan you pick I think you'll be pleasantly surprised at the speed difference over ADSL if you go with one of the faster plans (which by the way are half price on the speed boosts at the moment)
Hope this helps!