If you still need your ex-custom feeds in your Feeds Reader, please go ahead and add them back again to any category you like, or create a separate category for them.
If you use custom feeds in your posting campaigns or content widgets, they will be automatically updated. There is no further action required from you. The same is with your advanced filters.
But if you use custom feeds somewhere outside RSS Ground, you need to replace them with the new personal feed URLs. Copy related personal feed URL in Feeds Box: