Hi jarney, nothing wrong with not taking the paint off but they will need at least a light sanding if you want the new stuff to stick properly. The new paint will need something to adhere to otherwise it may start peeling. If they are magnolia then as Count-A-Strong says it could take 2 coats to cover properly. If you do decide to use 1 coat stuff I would wipe the skirting boards down with a tack cloth before painting as this will pick up all the small bits of dust on them that you won't see until you have put the paint on by which time it will be to late to so anything about it.