How hard (Time consuming) would it be to switch from RTF to DOC
RTF is the documentated part of the DOC specification.
The smarter decision would be to switch to HTML 5.0 and CSS 3.0.
Every layout engine I've seen has been either proprietary, or GPL or LGPL. The latter two licenses are incompatible with the e-Sword license. Proprietary engines usually require a licensing fee.
and would it be worth it to eSword users if Rick Switched
As a presentation markup language, I don't see any virtues in switching to DOC. Or, for that matter, ODF.
I do see some virtues in switching to OSIS, and ThML. The downside of switching to those to formats, is that, AFAIK, Rick would have to write the layout engine from scratch. Given the known issues with the ThML and OSIS specifications, writing the engine will be, from a programming perspective, extremely non-trivial. Furthermore, it is non-trivial to adapt those formats to usage within a database.
I see a greater virtue in switching to HTML 5.0, and related standards. However, I am not aware of a BSD licensed web engine. Whilst writing one from scratch is possible, maintance and bug-fixing could be distractions from working on e-Sword as a whole.
jonathon