Admissions people usually consider recent and consistently good college work to impact more in their decision than older poorer high school grades. So a high school slump can be overcome with good grades in recent college credits. Another consideration is that one should take multi-part science courses all at one college. Take all 12 credits of engineering physics at the same school. The reason is that the chapters covered are not guaranteed to be in the same order, so you could miss parts of the course and the new teaching environment can also take its on you.
Grades mean a lot.
Some professional majors have higher G.P.A. entry requirements than general admission requires. For example, education may require a 3.0 after 4 semesters in order to enter or continue in it. Likewise, business, pharmacy, special education frequently have internal admission standards set to limit enrollment. Yes, Mid-year transfer is usually available. But, not at all colleges and not in all majors.