If a price set is changed in terms of prices etc will that affect reports for past events that used that same price set?
If it did do that we would have a huge list of price sets just to maintain reporting!
Almost certainly no, but it might depend on the report you're looking at. Specifically, financial transaction data is stored with the full amounts, not just a reference to the price set, so you can't change a contribution by changing the priceset it came from.
In other words, it's safe to update a priceset from one year to the next for an annual event, for example.