its not ideal, a proper regas removes the old stuff, applys a vaccum to check for leaks, then refills, along with the right amount of lubricant.
I got my last one done at kwak fit watched em while they did it, and its automated so even they cant get it wrong, they hook up the pipes, and press go, think it was £35.
Proper regas removes any moisure from the system before refilling too.