A change in the definition of a crime is a major change, as it means you lose backwards compatibility that would allow you to compare earlier years. Such a change should therefore be done only if there is some extremely good reason for doing it. Rape itself is not new, and indeed almost certainly more ancient than human history. So what could be driving a change in our understanding of it, if it is not a change in the nature of the offense itself?
Let's look at the definitions.
Previously, offense data for forcible rape were collected under the legacy UCR definition: the carnal knowledge of a female forcibly and against her will. Beginning with the 2013 data year, the term “forcible” was removed from the offense title, and the definition was changed. The revised UCR definition of rape is: penetration, no matter how slight, of the vagina or anus with any body part or object, or oral penetration by a sex organ of another person, without the consent of the victim. Attempts or assaults to commit rape are also included in the statistics presented here; however, statutory rape and incest are excluded.So there are two particularly significant changes:
1) The old definition stated explicitly that only females could be raped.
2) The old definition was interested only in cases in which force was used to effect the rape, whereas the new definition doesn't care about the issue of force at all. It is only interested in whether or not there was consent.
If the issue of prison rape is taken seriously, just the first change should more than double the incidence of rape in America. It would also put an end to a statistical anomaly: rape has heretofore been the only violent crime that women suffer more often than men, and that will no longer be true. (So far they don't seem to be considering prison rape in these statistics, as the rate jumps according to the definition only from 24.0 to 39.0, and not to ~50+.)
The removal of force from the definition won't change the statistics as much as the change that removes the restriction against men being considered rape victims, but it is still also a very significant change to the standard.