I found this really quite confusing. It is very difficult to tell which extensions related to which version.
For the benefit of others, the versions of the extensions may not match the version of AcyMailing so you need to be careful which ones you delete. For my site the extensions that I needed to keep were:
- AcyMailing 6.1.8 (component)
- AcyMailing subscription form 1.0.0 (module)
- AcyMailing - Joomla integration 1.0.0 (plugin)
- AcyMailing - JCE integration 1.0.0 (plugin)
As jvstratum said you can reinstall AcyMailing 6 to ensure that you have all the correct plugins. However, please be aware that if you delete any of the above modules you may lose some of your configuration. I discovered that I had lost the subscription modules that I had created by inadvertently uninstalling the 'Acymailing subscription form' module.
You will probably also want to remove all of the data in the database that relates to version 5. This doesn't happen automatically so you have to do it manually. You can do so by deleting the tables that start with 'acymailing' (for version 6 they start with 'acym'). When you uninstall the Acymailing 5 component there is an alert with the SQL script that you need to run to do this.
I hope that this helps other people that are having the same problems!