PD12 is too old to qualify for the upgrade version and price. You therefore have to pay the full amount for the New User price. As a consequence you can keep the old version or uninstall it if you wish.

Quote My previous PD12 was not unistalled by PD18 and is functional. Uninstalling PD12 at this point could cause any problems? Thanks.

Not really, what needed to be uninstalled was. I have several versions (incl 12, 17 and 365) on my computer just to be able to try different things.

Edit : BTW - you may lose some of the features in PD 12 that are not available in PD 18. but do work in PD 18.

If you have the computer space, keep both active.

2nd edit; I read the original post as Rolando has purchased PD 18 and installed it. The question relates to parts of PD 12 not unistalled when PD 18 was installed. .
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Anything I post unless stated with a reference is my personal opinion.

In my opinion, upgrading rather than performing a clean install of the new software is always a mistake. By upgrading, you save a small amount of money and risk a large amount of stress. I have clean versions of PD17 and PD18 Ultimate on my computer. Both work very well. PD17 is patched and very stable, and PD18 will get the usual 3 or 4 patches over the next six to nine months. Then if I choose to get PD19 Ultimate when it's released a year from now, I'll uninstall PD17 at that time but keep the patched PD18. By keeping the fully patched version of the previous software, I've yet to have any interuption to my editing when installing the new offering.