Redmine

I have version 0.7.3 and working perfectly.
How can I migrate upward, and let the still running version 0.7.3?
That is, I want to migrate to version 1.8 on localhost. Meanwhile the version 0.7.3 will is in use on the server.

How can I have two Redmine running at the same time? A server (0.7.3) and the other Redmine 1.8 in localhost. So after testing in version 1.8, migrate all data from 0.7.3 to 1.8.

I know I need to migrate one at a time, until you reach the 1.8. Can I do it on localhost, as I leave the 0.7.3 running on the server to not affect the data updates in the company.