I want to add the validation support to existing insert and update forms that I already have created. With all the changes that have been made in the last year, would I be better off starting from scratch? How have others worked with their existing forms? I'm particularly interested in adding the feature of checking to see if the record already exists.

Hello Hope,
You can apply client-side validation on any of your existing forms.
Server-side validation ( like check if record exist or not) requires your database query and server actions (insert/update/delete) to be created, through Server Connect.