Probably a repeat of how Obama handled things.
http://www.huffingtonpost.com/2012/04/25/obama-marijuana-rai...
Not to mention that'd you'd be putting tens of thousands of people out of a job if you tried to "crack down" on one of the fastest growing industries in the country (where it's legal). Job creators.
Subject to the applicable statutes of limitations, as long as the federal laws are on the books, yes, people who committed offenses at a time when the those offenses were not prosecutorial priorities of the executive branch could be charged when the prosecutorial priorities of the executive branch change. That's sort of the key distinction between "prosecutorial priorities" and "laws".
The only thing that stopped the dispensary raids on California dispensaries was discretion. The law hasn't changed substantially enough that what they're doing is still considered kosher to the federal government.
As it stands, the next president, whatever his party, could simply decide to resume those raids, and would be perfectly within their statutory purview to do.
That said, I agree with the assertion that it is a states' rights issue, but SCOTUS precedent extends the power of the Interstate Commerce Clause to include much more than this, so according to current precedent, states rights only exist where the federal government expressly abdicates making a policy decision, which isn't something our federal government is terribly good at doing.