Try opening the workflow in the workflow editor. Warnings about what tools/tool-versions may be included in it, that are no longer available on that server, will be shown during loading. Then edit the workflow to resolve any of those issues (replace tools, reconnect noodles). In some cases, the workflow editor can repair a workflow without any action on your part (example: the warning will note that a tool was automatically updated to a newer version). Be sure to double check those changes to make sure the parameters and connections are still valid and save.
It is strongly recommended to do this on a copy of your workflow, not the original. That way you can start over fresh from another copy of the original as needed.
Thanks! Jen, Galaxy team