That complexity is illustrated in the DWP information booklet which provides guidance on deferrals. It runs to 60 pages and then recommends after all that that people get independent advice before making their decision. Even so, given the factors and variables, there is no guarantee that such advice would be forthcoming.
Reverting back to the class 3A distinction, that is clearly being directed at existing pensioners who currently get existing increments as a lump sum, so they are within the old system. It is being directed at people who are in the existing system rather than those in the single-tier system.