Let’s say I bought a $90 1 year subscription. If I’m on the last day of my subscription (right before it expires) and switch to lifetime, am I credited $90 towards it?
No, you’d be credited a prorated amount based on your unused subscription whenever you upgrade. So if you used half of a subscription and then upgrade you get half of your current level value toward the higher level.
So upgrading on the last day would be not much, if anything.
There’s usually a Christmas sale, so if you get the $90 yearly now and then by lifetime in December you would have only used 8 out of 12 months. So the last 4 months ($30) would be taken off the price off lifetime.