You'll need your own developer account, which shouldn't be a problem because they are free now. But basically if you have the source code, you can simply re-build it with your personal provisioning profile. Because you won't be able to set up an explicit application ID, you will simply create a wildcard id on Apple's developer site, then create a developer provisioning profile using that wildcard ID and your personal certificate. Then simply build it to the device directly.
Why are you not updating your device to iOS 9? If properly written, your app will work fin on iOS 9 if it worked on iOS 8. The opposite is not necessarily true, as an app with a minimum OS version of 9 probably makes SDK calls that did not exist in 8. Although some changes may be needed, you should be able to build your app with the latest version of Xcode even though you originally coded it in Swift 1.2. You can find simple instructions on how to do that upgrade. This will also allow you to address any potential iOS 9 issues.
If you don't have the source code, but have the .ipa file, you could also resign it using the instructions here: https://stackoverflow.com/a/25656455/3708242
This is not as ideal, because if there are iOS 9 or iOS 10 issues, you will not be able to fix them. I think keeping the source code available (if you are legally allowed to) will be your best bet.
