How to deal with missing properties on nodes from earlier versions

Hi guys, imagine the following scenario:

I have a custom Template, let's call it my-template. This template is used to create Pages with properties called propA, propB and propC. My Sling scripts under /apps/<...>/components expects the Pages under /content to have those properties.

This is deployed to production environment and everything works fine.

Now, the business needs changes and I need to change my-template. It now has properties propA, propX and propY, where: 

• propX is the same of former propB, but the previous name doesn't make more sense;
• propY is a brand new property and,
• propC was deleted as we don't need it anymore.

Then, my questions are:

1. How should my Sling scripts deal with the old Pages (the ones with propA, propB and propC) created before the new Template properties where there? 
2. Should I write my code to deal with both "versions" of Page I might have in the production environment?
3. Having a relational database background, I would run a update SQL script to fix the old data. But how can I do such a thing with JCR? And more, Am I supposed to do such a thing?
4. And lastly, how about nodes that are not under /content (e.g. /etc nodes)?

I  hope I could get my idea through. Shall you have any question, please, let me know.

Thank you in advance.