thats only for the situations where you have 7 years of PS+ and 0 of PSNow. In that case, if you want to upgrade to PS+ Premium (tier 3) they would want you to pay a crazy fee to keep your 7 years, they are really foggy on conversion rates and shit. Or you can just stick to 7 years of PS+ Basic (tier 1) and nothing at all changes for you. It is likely you won't be able to just buy 1 month of PS+ Premium and then dip back to PS+ Basic, Sony seems pretty shitty at moving back and forth between tiers.
If you were semi-smart and currently have years of PS+, you just need to activate 1-2 months of PSNow and then you will have years of PS+ Premium (tier 3) converted automatically. Any combo of PS+ and PSNow will convert you to PS+ Premium for whichever of the two is longest.
So for example I have 6 years of PS+ remaining and 2 years of PSNow, and I will convert to 6 years of PS+ Premium on june 13th (USA release) because of the PSNow. I could have just had one month of PSNow and it would be exactly the same. If i had 0 months of PSNow I would be sorta fucked though.
tl;dr if you have anything more than 1 year of PS+ RIGHT now you either need to be cool with never getting PS+ Premium OR you need to get your hands on 1 month of PSNow asap.