Unless you are (or someone is) getting paid to update, maintain, and upgrade this frequently (I would suggest every six months) I do not believe this is something than can be easily accomplished. I would recommend, if you are a contractor, that you create an ongoing maintenance agreement that provides deployments tests every six months. If you are an employee, I'd recommend you attempt to put into place such a program with your employer.
If people do not want to pay to maintain this, then it's probably not going to work for 25 years despite your best efforts. If they only want to pay to maintain it when they REALLY need it to be maintained, it's going to be expensive and not necessarily possible. I see this kind of stuff all the time - customers don't want to pay to maintain something, until it breaks, then they want to pay.
As an example (that isn't related to embedded, but the principle is the same), I JUST ported over Google OpenID logging in to OAuth2 in a PHP system. This was way more expensive, and way higher stress, because the customer did not listen to me six months ago when I said Google would no longer be supporting OpenID, and we should migrate to OAuth2 or they wouldn't be able to login. They didn't listen to me then, but called me when they couldn't login. The hourly rate was higher, because I hadn't scheduled this work, and it took longer (billable time) because I was under pressure to get the calendar time as small as possible. However, we got it! There are examples all the time where things basically need to be scrapped, since the technology is so, so, old and the provider doesn't exist anymore. For example, I am having a lot of trouble finding documentation for CouchBase 1.0 on some CouchBase work a customer wants done.... This wouldn't have been an issue if they had kept upgrading CouchBase versions along the way. Now, it's a pain.