As far as I'm currently aware, we don't have any active plans to require customers with non-SSD VPSes to upgrade to a SSD VPS. If this does ever change, we'll let you know well ahead of time.
Keep in mind that there are several differences between these plans:
[*] The old VPSes are hosted on machines with magnetic (or "spinning disk") storage. The SSD VPSes are hosted on machines with SSD storage, which is considerably faster.
[*] The old plans start at 300 MB of memory for $15/month, and go up to 4 GB of memory for $200/month. The new plans start at 1 GB of memory for $15/month, and can be resized up to 8 GB of memory for $120/month. In other words, you get about three times as much memory for the same price!
[*] The old plans include unlimited storage, and are subject to our Unlimited Policy. The new plans include 30 GB of disk space for every 1 GB of memory, and are not subject to the Unlimited Policy. (That is, you can store data on these VPSes that would not be permitted on an older VPS or a shared hosting plan, such as personal backups.)