The DIR-628 router can only do 2.4Ghz or 5.0Ghz one at a time. When you change from 2.4Ghz, all the existing rules (access rules, port forwarding, passwords SSID etc...) remain the same. However, since you are changing the "radio" to 5.0Ghz, the channels change.
Please note: Changing to 5.0Ghz "radio," you would need to make sure that the wireless adapters on your PC can also use the 5.0Ghz "radio" first. Not every Wireless N adapters can use the 5.0Ghz range, unless the manufacturer states "clearly" that the adapter does work.