How hard is it to change universities after one year in a graduate program (USA, PhD in Chemistry)? Let's assume I am going to a University for a PhD program in the  US. After attending for one year, I feel that it would be better for me to switch programs and University. How hard would it be to do that? I'm an international student not yet admitted to a graduate program.
In chemistry (or physics) it would not necessarily be hard if your 1st year grades were great and you made progress in other activities during that year that showed you would be successful in US PhD system. The application timing is the difficult issue, as you apply in the fall to new school and you would have had no convincing track record by fall the first year.

People are accepting of helping (recommendations, etc) a good student switch if it is clear that no group within the department is suitable for that student due to funding, or subfield interest.

However, your email appears as if you are asking how to work the system to apply to one school  to get into the US, but then go somewhere else? Faculty are not clueless and some will be pick up on what feels like a lack of good faith or transparency during your year or two at the first institution. It will not lead to faculty being helpful. (And some resentment if it felt that scarce resources as TA or RA slots were 'wasted').