There are a few variables to consider. The easiest thing is matching the salinity when mixing the new salt. The bigger picture is the makeup of the salt in regards to CA, MG, ALK. Switching from something like Instant Ocean to Red Sea Pro will have adverse affects as Red Sea Pro is more "potent" salt; meaning it mixes at elevated levels because it is intended for heavily stocked coral reefs. You can either slowly adjust over time as suggested or find something similar to your current water parameters as use that. If your current ALK is 8 dKH with Instant Ocean and you do a massive water change with Red Sea Coral Pro, your ALK is going to jump big time and has the potential to stress your coral (assuming you have any).