What happens with current delegates if presidential candidates drop out of race?
Can Rubio, Kasich, etc delegates be assigned to Cruz before the convention? Or vice versa?
It varies by state, but, in general, a candidate who drops out can't 'give' his delegates to another candidate. Delegates have to follow their state party rules.
For example, Iowa delegates for this cycle have their (first ballot) votes automatically allotted whether or not the candidate qualifies to have his name placed in nomination or whether he has dropped out.