[schapplm]

Should one be able to use WatchedList to migrate videos marked as watched in Exodus from FreeTelly 16.1 to Kodi 17.1?
[...] json requests suggested earlier in this topic to webserver return empty on both v16 & 17

I think this is not possible, since in the installation notes for Exodus it says "Library integration is not supported, but the following are: [...]".
So since it does not use the Kodi video library, the json queries return empty and WatchedList does not work.
The problem here is the addon Exodus, not FreeTelly itself (which seems to be just a Kodi fork).

If no, how to get previously watched in Exodus into newer version of Kodi?

This is not possible unless you do some coding yourself, e.g.

• write python scripts to loop through the Exodus data and feed this to the watchedlist database (look at the code of WatchedList)
  
• script or addon to go through the data of one of the addons supported with Exodus (like trakt.tv). I would not recommend to use an online service to keep track of watched pirated movies though.
  
• write an addon for integrating Exodus into the Kodi video library (would be blacklisted like Exodus)
  
• write an addon for integrating Exodus into the WatchedList addon (would be blacklisted like Exodus)
  

Since Exodus is banned from the official Kodi team due to support of piracy, we can not give any support on this forum. You have to go to the Exodus/FreeTelly forum.