Joining 3/16in copper tube with 1/4in plastic tube

I have a pneumatic control system that am currently in the process of renovating. The existing system is 3/16in copper tube. The problem is a mistake was made and 1/4in plastic tube was run all over the area, and the walls.

I am hoping I may be able to just slide the plastic tube over the copper. But just in case that doesn't work out, is there a sort of fitting that I could use?

Thanks!

Well typically I'd use a Swagelok SS-400-6-3. but you can also get non stainless fitting as well by changing the SS to something else as per the manual. 400 means 1/4" -6 means union. -3 means reducing to 3/16"
google Swagelok fittings manual for help

http://www.stcvalve.com/Push_In_Fitting.htm?gclid=CKrto83issoCFY9gfgodjAQHMA

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