During nodule formation Rhizobium is converted to bacteroids. What's the difference between them?
Bacteroids are the endosymbiotic forms of rhizobia (in the root nodules of leguminous plants).
Löhnis & Hansen (1921) J Agric Res 20: 543-556; emphasis mine:
It is not superfluous to emphasize once more that persistence in calling these forms "bacteroids" is by no means to be recommended. They are true bacteria, not foreign bodies looking like bacteria, as Frank's pupil Brunchorst erroneously believed. To speak of a "bacteroid" growth of bacteria is no less absurd than it would be to speak of a "fungoid" growth of fungi.
Gerson & Patel (1975) Appl Microbiol 30: 193-198; emphasis mine:
Rhizobium is an aerobe, existing as a free-living bacterium in soil or as a partially anaerobic symbiont in the root nodules of leguminous plants. It is in the symbiotic form (bacteroid), which is frequently morphogenetically different, that nitrogen fixation takes place.
MacKenzie et al. (1973) J Bacteriol. 113: 387-393; emphasis mine:
[C]hanges [accompany] the transformation from the free-living rod forms to the nitrogen-fixing bacteroid forms. (...) During the transformation into the bacteroidal state the wall decreased in thickness, became less rigid, and developed a particulate surface.