I use the Transit Capacity and Quality of Service Manual (TCQSM) coupled with historical data for preliminary planning type projects that include, demand, supply, level of service, etc.
Unfortunately, you're delving into transportation/traffic/transit engineering and planning territory at this point. Unless you're doing an academic or research exercise, transit planning is very complex and simply projecting historical data without considering socioeconomic and spatial considerations, regional and local plans will yield inaccurate and false results.
If you're doing an academic exercise, then I suggest, you look at the zoning of the areas by category, learn the TCQSM as much as you can and play around with the results.
From what I understand, you are comparing two or multiple scenarios:
- Scenario A: keep the current service as is
- Scenario B: Change a portion of the service
- Scenario C: reduce frequency/time of the service
Each of those scenarios will have to consider multiple issues, such as the 0.25 mile rule, or if a change in time or location happens, how many will spill to other routes, so if you reduce Route A by 5 riders and hour, you might increase another by 4 so the net loss is 1. Detour times and parking availability are also some considerations. the TCQSM outlines these nicely. In contrast, if the re-route is around an area with low ridership, you might have low net increase in short run but you might have an increase in the net ridership in the long run, if that area gets developed or more transit riders move there.