I had a similar problem in another game, upgrading Saab340 to 2000's. I adjusted the turn around time so take-off time didn't change. It doesn't work for all flights but can get the new Saab2000's into use and earning money rather than replacing old rental 340's with more 340's.
If your destination has night closures this can work well for you on some flights, with a later departure time from your base airport, and an earlier arrival of course. It does unbalance that aircrafts schedule so you need to be ready to add an extra aircraft and shuffle your schedules to keep flying hours high.