I have Sonar X3 Production, and although the store is now closed, is there any chance of up grading anyway?

Hi. I doubt it.
You could try some 3rd party sellers, and see if they still sell it.
All the best.

Check out deals on Studio One for Sonar users.

Some links here, but not cheap :
https://www.bax-shop.de/cakewalk-sonar-platinum-producer-software-download?gclid=Cj0KCQiAvrfSBRC2ARIsAFumcm9CazchVH-jeEDCq6v0dardO62zky9ldKwBPF7wu6S_4E5fE6ceBBQaAvjiEALw_wcB
 
https://www.amazon.com/Ca...Software/dp/B00TFTVXEE
 
I recommend to phone the seller .

You might buy just to find out it cannot be authorized and will then be asking for your money back. 
There is thread where you can still get Home Studio free with a magazine subscription. THey seed to manage to get it working but seemed complicated. 
 
Be happy you have X3e. It is as good a DAW as many. 
You'll be disapointed in most of the other DAW's you can certainly try the demos like many of us did and returned to Sonar. X3e will work for you untill you no longer have a Computer that runs a supporting OS= 10-15 years from now,  unlike the versions that came afterwards which are doomed unless they unlock them from needing the server. 
X3e is where I will go before I switch to another DAW if Splat goes down. 
There really is no reason to spend more money unless you really don't like what you have or feel you need a new toy. 
 

The prices are going thru the roof!

I followed those links and it looks sketchy to me as they seem to be saying it's a disk in a box?? 
You should read the reviews. Gives you a huge insight to how the real world conceived Sonar's  learning curve and support. I like like the guy who asks if you can record vocals...

All is not lost.
You can get this DAW from Cakewalk right now.
https://thesoftware.shop/...udio-giveaway/#buy-now

That seems a bit fishy. 

I would wait for more solid information and options. Why waste your money?