Some Thermador ranges are not field convertable from natural gas to LP. The PRG364EDG falls in this category. If you have one that is not convertiable; you will need a different range.
The PG364GEBS is not a listed unit at Thermador; I've found it for sale at AJ Madison. Since that vendor doesn't list the conversion kit (and they sell ones for other Thermador ranges), I do not believe this is a field convertible unit.
Thermador has manuals and installation guides for many of their appliances here: http://www.thermador.com/trade/specifications-guides
. Most Thermador ranges come with the installation guide, as well as the owner's guide, if you still have your documentation. Otherwise, try the Thermador site for the documentation (check if the company uses a different part number internally). If the conversion kit is available, the part number will be provided in the section on the Gas Requirements and Hookup. If it isn't, the installation guide will clearly state that fact.
Other models are convertible. The PRD304GHU uses the conversion unit:
PALPKITHC. You will need to confirm your model since the number of
orifices will depend on the number of burners and the low simmer
feature. (The PRD304GHU is also a 4 burner with 2 of them extra-low
simmer burners model.)
For units that are convertible, you can usually order the part through the same stores that sell the appliance. Alternatively, have your gas service installer order the parts. The installation guide for the conversion kit will be included in the kit. For most ranges that are field convertable, the regulator converts from natural gas to LP by rotating a cap in the regulator by 180 degrees. (Take out the part, turn it over so that LP shows and reinstall.)
I hope this helps and I wiish that I had better news for you.