Our SFDC has been used a bit back to front so we have a lot of contacts when they're actually leads. I want to go through each one and convert some back to leads (and delete their contact details). Does anybody know how to do this?
This sounds like a one-time activity. I would suggest you run a report (or use Apex Data Loader) to extract your Contacts (along with the Record Id) into Excel. Go through that and figure out what you want to convert and delete, put that into a CSV file.
Then using Apex Data Loader you can load the file as an "Insert" into the Lead object and once you've verified that was successful you can take that same file and load it as a "Delete" into the Contact object.
Because Leads and Contacts have so many similar fields it should be easy to map the Contact extract into the Lead object.
You can export your contacts and import them as leads, but you should only do so if they are leads. Leads are those people you want to follow up on because they might lead to a sale. A contact can be anybody-- a past client, a future client, a current client, a contractor, a vendor, an employee, and so on. I strongly recommend that you avoid "deleting" these contacts, because they're still contacts. You're still in contact with them. You might want to also review the training videos on Leads.
What you want to achieve is possible with an app I recently built and got approved on the App Exchange, Inverto360. This Account/Contact to Lead App is now available in the App Exchange