We have only recently decided to jump on the ADHD med bandwagon and are still in the process of finding the right medication for my oldest DS.
We're currently on med #3, and after a week, I can already tell that we either need a dosage adjustment or a med #4.

It's more than a bit disheartening, especially b/c my DH, who is also on meds for adult ADHD, has had such an easy time of things -- no negative side effects, correct dosage and med combo from day one -- that I naively assumed DS would react similarly. Then again, DS isn't on the same meds as DH (he's tried Adderall and is currently on Vyvance -- both worked great), and while his psych explained that DS's neurochemistry was different as an adolescent and that's why we're not going there (yet), I'm beginning to think we might be better off trying something I know works for another family member, yk?
Anyway, sorry for the sidetrack there. In both cases thus far, switching meds has not been difficult; in fact, b/c the meds weren't working as they should, it was a relief to stop/switch. We stopped the previous two meds for between 1-3 days before starting the new med and while DS clearly acted unmedicated during that time frame, he also ceased having all the horrid side effects he was experiencing, so it ended up being a positive.
With that said, I'm sure different meds have different guidelines regarding stopping and switching, so you'll want to check with a dr.
Thus far, DS has been on:
1. Concerta (stimulant) -- the dosage was too low to have any effect on behavior, but DS experienced daily stomach cramping and diahrrea with this med, plus zero appetite. DS is not in the least overweight, so he really can't afford to stop eating!
2. Dextroamphetamine (stimulant) -- this med and dosage worked beautifully to control ADHD symptoms, but despite adding a probiotic to help with the digestive issues, DS continued to have daily severe bouts of stomach pain and could barely eat anything.

Major bummer.
3. Strattera (non-stimulant) -- this is the one we're currently trialling. No side effects, hooray! But unfortunately, no effect, either -- DS's behavior since starting on it has been, in a word, horrific. Impulsivity through the roof, figdeting out of control, accidentally destroying everything in sight. Sigh.
He's a great kid, and I really, really want to get this sorted out before school starts in a few weeks from now, so that his school year can be successful.
I understand your hesitation to switch, but if she's habituated to the med, then having her continue on with the status quo won't work, either.
Good luck and let us know how it works out! It really helps me to know what meds are working for different kids b/c we have yet to find the "right" fit.
Guin