1. The usual price for "standard" membership is $120 and the usual price for "plus" membership is $180. Anyone who registers after 15 November will pay $120 or $180 per year, every year. (The "usual price" can be less or more, based on various other factors, e.g. whether you make recurring payment, whether you pay by PayPal/credit card or some other method, whether you have to pay VAT, etc.)
2. People who pre-register before 15 November will pay only $109 or $149 per year, every year. So, the $11 or $31 discount (if you pre-register before 15 November) is not only for this year, but for every year. Right?
Correct. If you click on the purchase/register button for either package, the first screen you see will show you what you will pay now and then yearly after that. This does not mean that pricing cannot change in the future, but if it does you will be notified beforehand. See http://www.proz.com/professional-membership#faq
3. Do I assume correctly that if I were to cancel my membership at some stage, and then resume my membership, then I would no longer be on the discount, i.e. that when I start paying again I would be paying the full price from then on?
That is correct.
4. And if I have been on the plus package for, say, 3 years, and then decide to switch to the standard package, then will I have to pay the non-discounted price for the standard package from that point onward, or will I still qualify for the discounted price even though I've switched packages?
You would pay the current price of the Standard package at that time, yes.
5. Oh, I see everything is in USD and I don't see EUR anyway. Is everything going to be in USD from this point onwards?
6. Will there be reminders shortly before the recurring payment that a payment is about to go down, so that I can ensure that there are sufficient funds in the account for the payment?
Yes. You can also see information about your membership, subscription, renewal dates, etc. by mousing over your name at the top of the site and clicking on "Manage membership."
7. If I cancel my membership, will my membership stop immediately or only at the end of the billing cycle (which I assume is 15 November each year)?
For Standard packages, yes. Plus packages may take longer to become inactive. See http://www.proz.com/professional-membership#faq
8. If I switch from standard to plus, or vice versa, will I have to wait for 15 November of that year before the change becomes active?
No. After the release of the Plus package on November 15, a switch from one package to another will happen when you choose to make the switch.