As far as i know, this is not possible. Your only option is to go back to the latest revision that was still 8.18.1 and redo all your changes.
Using svn repository you can download specific revision or you can create new branch using specific revision.
As Javith mentioned you could reverse merge (undo-ing commits). But the reverse merge capability in Studio pro cannot seem to deal with cross-mendix-version. The only way I can think of is:
In an other post you say that Excel Exporter works in Mendix 9.1.1
Does that mean that this post is no longer relevant?
Hi Maxim Velichkin,
You can go to Mandix Studio pro in the market place then select the option “other releases” there you can download the old mendix version 8.18.1