The ring on these types of bottle caps is usually attached with several strands of plastic. This specific type is attached quite strongly and many people lacking strength in their hands find it difficult to open them.
On the inside of the cap ring there is a row of "barbs" that catch onto the bottle nech while twisting the cap and cause the ring to seperate fron the neck. If you manage to deactivate those barbs, you might be able to open the bottle without seperating the cap ring.
Please note that this method may fail despite your best efforts.
I would heat the cap with a blow hairdrier and then insert a thin but sturdy object between cap and bottle neck to press the barbs flat to the cap ring. That way they hopefully don't catch on the bottle neck.
Since the plastic is slightly more elastic while hot, I would try to carefully open the bottle while the cap is still hot. Be aware that soft plastic usually loses it's elasticity over time.
An alternative would be to store the bottle in a preassurized chamber for some time. The plastic of the bottle itself is permeable for gases, hence why the bottle collapsed. In theory you should be able to reverse the process and press gasses back into the bottle, but it would probably be a very long and slow process. I'm not sure how practicable it is to keep a pressure chamber pressurized for half a year or longer and I honestly don't know if this would even work.