If you are able to open your projects, then you have the correct version without updates. If you had done any updates you would not have even been able to get to the first screen, so you can continue editing and everything that was there before should still be there.
As far as upgrading is concerned, I cannot personally comment as to how effective a DXF output from V15 to V2017 would be, as I have not done it but it is obvious there would be a lot of work reconstructing and refurnishing to do and I suspect that the DXF export would not be editable in V2017.
If you were to go to Personal Architect you would find that it will open your projects exactly as you had them in V15 and you can also set PA to use your V15 catalogue if you want, so any customised or imported objects would still be there. This would save a HUGE amount of work as V2017 would almost mean doing your projects all over again.
Both programs have their pros and cons of course. TFP V2017 has some nice automated features that will appeal to some and it renders very quickly, but, in my opinion, not with the realism of TFP. Envisioneer/PA has extreme precision (to 4 decimal places) and the photorealistic renderings are just that - they look like real photos! My main use of these programs was for photorealistic renderings which is why I went from V16 to Envisioneer (the parent program). It opens all my existing projects (dozens of them) and continued to give me the exact control and accuracy of lighting from the wattage, angle, type and colour of the light bulb, to the exact sun position at any time of the day, month and year, in any city anywhere in the world. Some do not need that precision, so may be quite content with V2017.
Both programs are very good and the same price, so it is really up to you which way to go, depending on your needs and expectations.