I’m afraid at this time account revaluations need to be performed manually using the process described here. These revaluations are typically done at the accounting year-end but can be performed more frequently if you prefer.
For now I have switched your post to a “Feature Request”. We are happy to explore some options for automated revaluation journals, it would however be great to see some additional support from other users for this so we can set a priority.
EDIT:
Regarding the ability to preview exchange rates, this is more likely to be something that we can address in the short term. It may also be possible to create a simple revaluation tool so you can see any +/- changes and quickly revalue without needing to create journals.