I agree with meme.
I had to read what you’d written a few times to properly understand it (I think) so, if I’ve got this wrong, please correct me.
You saw a different neuro today? You’d originally agreed to start Rebif and move onto BG12 when it was approved, but this second neuro is saying that, while you can still do that, you might be better on Tysabri?
I think the more serious neuro disorder might be a bit of a red herring - I don’t think they would start you on a DMD unless they were sure it was MS. For now at least, it might be best to ignore it.
I would guess that the neuro believes your MS to be “highly active” / aggressive. If that’s the case, then most neuros would want to put the patient on the best available DMD and, at the moment, that’s Tysabri. (You’re looking at an ~81% reduction in relapses versus Rebif’s best ever result of ~46%, plus it significantly slows progression whereas Rebif only moderately helps.)
As meme has said, Tysabri comes with the very small risk of PML. The JC virus is a virus that about 54% (I think) of the population carry, but which the immune system keeps dormant so we don’t normally know anything about it. Tysabri suppresses part of the immune system and this can allow the JC virus to become active and lead to the brain disease PML which, if it goes unnoticed for long enough (months, not weeks, but I can’t remember exactly), is fatal. Once contracted, PML can only be stopped by coming off Tysabri, having it flushed from your system, and allowing your normal immune system to reboot. The damage already caused, which is similar to MS symptoms, is permanent. PML is the reason that people on Tysabri are monitored very carefully - any sign of any virus and the monthly Tysabri infusion is delayed as a precaution.
The chance of getting PML is effectively zero if you do not carry the JC virus. The blood test the neuro referred to will be the blood test for the JC virus, which everyone on or being considered for Tysabri has to have these days. Unfortunately, although the JC virus is normally contracted in childhood, it is possible to catch it as an adult so the test has to be done regularly and whether or not to stay on Tysabri considered. According to the latest data, the risk of getting PML is highest if you are JCV positive, have had steroids at some point and have been on Tysabri for between 2 and 3 years. This (highest) risk is approximately 1%. You can get more info and see the stats here: http://multiple-sclerosis-research.blogspot.co.uk/2012/12/natalizumab-pml-risk-stratification.html
It’s a difficult decision. There is no doubt that Tysabri is the best DMD available at the moment. Unfortunately, that comes with a very scary potential side effect 
It’s worth knowing that both Gilenya and BG12 have caused PML. It’s not as common, but the risk is still there.
If I were you, I would wait and see what the MRI reveals and take it from there. No point in worrying about things that might not happen!
Karen x