"Transform" is better to use there. If I were writing this sentence, I'd like it this way:
The moment the amalgam filling left my mouth it [was] transformed into hazardous waste, only legal on public streets when contained in a hazardous waste container.
In any case, we don't say that a filling would "drive on public streets." You can make it work, like I did, without using any verb there. And if I were more educated in the terminology surrounding hazardous waste disposal, I might be able to improve on the term at the end of the sentence.
Also, when you add the preposition "into" (instead of "in") after the verb you choose, it has the effect of emphasizing that a change or transformation occurred.
Other options you could use in place of "transform," which is the one I like best, are:
The moment the amalgam filling left my mouth it:
was converted into hazardous waste
metamorphosed into hazardous waste
changed into hazardous waste
became hazardous waste
turned into hazardous waste
mutated into hazardous waste