Streetcar numbering has run the gamut over the years. Before the semiconvertible era, pretty much anything went. The semiconvertibles (Type 1 thru 5) were numbered consecutively starting at 5000 and ending at 5970, except for a gap between Type 4 5475 and Type 5 5500.
Center-entrance cars were 6000s (motors) and 7000s (trailers). 4000s were used first for homebuilt "two rooms and a bath" articulateds, and later for secondhand ex-Eastern Mass Street Railway semiconvertibles. The 9000 series was used briefly for Birneys.
The "modern" era has used the 3000 series exclusively (so far). PCCs were numbered sequentially, regardless of design variations, starting with 3001 and ending with 3346. Light rail vehicles, of course, are not so neatly numbered, and there are gaps between each series, so that a 500-slot number series is used up on only 311 cars (144 Boeings, 120 Type 7s, and 47 Type 8s). Life is imperfect.
Now, it is true that there is some precedence to renumbering cars in order to stay within a certain number series... four cars in the Boeing fleet were renumbered before delivery so the 144 cars (out of 175) finally accepted ended up in a neat 3400-3543 series. I'm not sure why it was necessary, but it was only four cars, and it was done before they were delivered and accepted.
On the other hand, take 3875 as an example. It's already been accepted, it's in revenue service, and it already has a maintenance history and other records tagged to that number and lurking in a whole bunch of different computers. Renumber it, and you have to go back and change all of the records that refer to it. Sure, it can be done, but is it somehow necessary to have a neat 3800-3846 series? Doing that would require the renumbering of 17 cars currently numbered beyond 3846, 14 of which are already in service. I might be wrong, but I don't see it happening.