If they are plants that will do fine in your non-raised soil, I'd leave them. If they require good soil, I'd raise the bed. That's the usual reason for raised beds, bad soil (like our clay).

Is there any particular reason that you want a raised bed, or is it just that you have heard that raised beds are better?

I just enlarged a flower bed with perennials, and covered today with mulch.
Then I realized I should have added soil to make it a raised bed..

Is a raised bed all that beneficial? Should I dig up and re-do?
Will a raised bed make a difference for containing the mulch?
(Is that why professionally done flower beds are often raised?)