I was more concerned about installing a new instance of SQL Server (FULL) when an instance of SQL Server Express already exists and then realized that both can exist at the same time.
Given that there's now a high probablity that we'll have to bring the new box online with SQL Server Express, I was very worried about issues down the road with the full version. Unfortunately, there are some 'issues' with getting the full version on the box now. I would wait to bring the box online with the full version all at once, but there's a need to move to the new box.
My concern was that we'd have to take the box down for an extended period of time for the full version to be installed, but once the 'issues' are resolved, we can handle installing it over a weekend.
I see. Sorry I was speaking with the assumption that the two can coexist, which they most certainly can. Note also that you cannot upgrade an Express instance to a full install so you'll have a side-by-side or you can uninstall Express, then install full if you want a fresh start.
I have never had an issue adding a full instance to a machine with an Express instance already installed. If memory serves, a reboot will not even be necessary.
If you decide to add a full instance without uninstalling Express, once the Express instance has outlived its usefulness you may consider disabling the Express service rather than uninstalling it. I have heard of issues being created with full instances by uninstalling an Express instance.
__________________________________________________________________________________________________There are no special teachers of virtue, because virtue is taught by the whole community. --Plato