Retake the math GRE... as a grad student, preparing to transfer?
Among elite and maybe even top-20-or-so math grad schools, many use the math subject test GRE as an easy, automated filter... even while recognizing to some degree that it doesn't really test what we'd like... because even after applying that slightly unreasonable filter, there are still enough good candidates.

So, yes, if you have a really-mediocre score, there would be advantages in re-taking.

But/and, as in comments, letters of recommendation are far more important once you've gotten past some initial filtering. The letters should be from people who can be believed to understand what a serious grad program in math is about, e.g., by having gone through one themselves, and they should say that you'll excel. 

Meanwhile, in contrast to undergrad institutions, there really is no notion of "transferring" to another grad program in math. Nothing gets easier. You will be expected to have excelled in the year you've spent already, which it seems you have. You'd not get to really compete directly with people with fresh B.S. degrees, since, in effect, "performance/showing-potential" is prorated on what chances you've had, how much time you've had, to demonstrate it. This is reasonable, in many regards.

In summary: yes, re-take the GRE, to overcome initial filters. After that, it's about letters of recommendation, and (for me) your personal statement. That is, explaining that you got a late start (which can easily happen to sensible people) can account for some things. True, some admissions committee people might be looking for people who've "always known" that they wanted to do mathematics, but that can't be helped.