If it's not needed anymore, I'd also recommend to completely delete the Vlan. I don't see an advantage in switching back to "Single Vlan"-mode though. Especially when dealing with boxes as big as the MX450, chances are high that there will be a requirement for more than one Vlan sooner or later.
P.S.: There will be no outages for either of the options (at least from the outside point of view).