In the most basic terms, re-purposing an eVar (or a prop, or an event, etc) is taking that dimension which used to collect values for one thing (let's use an example of collecting the unique asset id for your content) and wanting to start collecting something new instead (let's pretend you are cleaning up an old implementation where you used both prop1 and eVar1 to collect the unique asset id, you don't need this in two places, so you want to start collecting something that will make use of the expiry more efficiently like an internal tracking campaign, etc).
So basically collecting values for A and re-purposing to value B.
There are probably lots of different thoughts on how to achieve this, this is my policy, but it's by no means the only way to do this.... I like to first turn off the tracking for value A and watch to make sure that all the old data stops and the eVar is empty (of course, people hitting cached versions of your pages may still send in a few random hits with the old values, there's not a lot you can do about that (unless the data is very easy to recognize - then you can create a backup processing rule to remove old values that may sneak in to keep your new data super clean).
Once I have confirmed the data has stopped flowing, I like to let the eVar sit unused for at least a few months (usually longer, I tend to turn off tracking when it's no longer needed, then later on reuse the old dimension when I need to create something new down the line), this allows our teams to get used to the fact that we are no longer collecting values for A any more, and it gives a nice separation between the different data sets. I will also rename the eVar to something like "Value A (Deprecated)" or "Value A (no longer in use") for a short time, then I will disable it a bit later if this is going to be a longer term change over.
Once I am ready to reuse it, I will first re-enable in Prod and check there are still no values - interesting fact, disabled props and eVars will still collect data, people just can't see it... so re-enabling it after months or a year allows me to confirm it's still good and clean and ready for re-use.
I will configure the settings, expiry, name, description, etc for value B, then start to do my testing sessions, making sure my values come in (and IF I used a processing rule to prevent "historical values from coming in, that my new values aren't triggering that rule - I don't want to throw out the new data - if the values are too similar, I may have to drop the processing rule).
When the testing is complete, and I am satisfied in our QA suite, then I make the changes in prod and the new Value B starts flowing and is ready to use.
