I recently converted two weeks under a similar promotion. If it works the way it did for us: there is no cost for the conversion, and you can "undo" the conversion later also for no cost. For me, that was an easy call.
Again, if it works the way it did for us: When you call back, you'll be assigned to someone in an office who will schedule a longer call. In that call, they will verbally explain the points program, the costs, and the benefits. We were already Wyndham owners, so that part was pretty easy. They will also confirm how many points you would get. The letter (probably) doesn't tell you that, but you can figure it out by looking at the point chart for the resort here:
Book your stay at Club Wyndham Kona Hawaiian Resort on the Big Island, HI. Explore the best Big Island attractions like the Kona coffee plantation and top snorkel spots.
clubwyndham.wyndhamdestinations.com
If it's enough points, at least 500k, that would make you Silver VIP.
The conversion results in VIP-eligible points only if you are the original owner of record of the weeks. If you bought them resale, the converted points are not VIP-eligible. Based on my experience, your account would not be enrolled in PlusPartners (unless you happen to already be Wyndham points owners with an eligible account). That was not an important consideration for me, as the only material benefit of that IMO is access to RCI Points inventory in your Wyndham account. But, you pay a slightly lower program fee without it.
You will pay a little bit more than you do now: a program fee that is added on to the maintenance fees you already pay, based on how many points you are granted. It will not be much: something between $165 and $200 each year, probably. That includes access to the Points system and the RCI account.
You will still have priority access to the resort: between 13 and 10 months, you will be entitled to your underlying ownership: this period is referred to as ARP, the Advanced Reservation Priority period. If you own fixed weeks, those are the weeks you can book. If you own floating weeks, you can book within your floating season, subject to availability, during ARP. The availability will come out of the set of inventory that has been converted. Unless you own a fixed unit, you will not be able to choose a unit when you reserve a converted week, even during ARP. This might matter if e.g. the views are drastically different within the float category you own, and you can choose now.
If you plan to go to your home resort "most of the time" (say, three out of every four cycles), the conversion might not make sense. And, it sounds like right now you are happy to just go back to Kona every other year. If you think you'd want to go somehwere else even half the time, then the conversion might be a good idea. The other big consideration is what your "fee ratio" ends up being: that is usually expressed in dollars-per-thousand-points. The numerator is how much you pay each year (this will be 1/2 your every-other-year maintenance fees plus the annual program fee), and the denominator is the points you own (effectively, 1/2 your every-other-year total.) Club Wyndham access is about $7.25. That's considered a little worse than average.
