It all depends on how much extra room you have on the rim with the 215's. If the rims are too small, then the 235's will bulge out the sides of the rim making it look weird. 

I'd look for rims specifically for 235's. Also note that increasing the wheel diameter by 1 inch will slightly throw off your speedometer reading.

Don’t even think about it! Your OEM tire diameter is estimated at 27.7 in., … your proposed size will be around 28.9 in.

And so you know: … you have to keep the diameter the same, … and width is limited by the available space under the fender, … allowing for full wheel travel as you drive. Think about that, …

Another thing, rim that accommodates 215 tire width is most likely not sufficient to accommodate 235 tire width. Plus offset needs to be considered if you change the rim, …

Anyway, it’s a phlunkin’ old Buick, … what’s your objective with this proposed tire switch? ( … just curious.)

F.S.

I agree with Razmig about the wider rims.

you definitely want to get an expert opinion about fitment before making that kind of investment.  My guess is that a Buick of that vintage will have plenty of clearance.  And you can look on Tirerack and Discount Tire’s websites; you put in your year make and model and they list alternate tire sizes that they say will fit.  But ultimately, you should take it in to a shop that has proper expertise and directly verify that wider tires aren’t going to rub before you buy them.

The biggest concerns with upping tire size are: rubbing, speedometer calibration, and aesthetics.

Going up to that side will throw your speedometer off by about 5mph. You SHOULD have clearance to notrun, but definitely have this verified by a custom shop. Do not count on tire stores to tell you the truth or know what they are talking about on this. 

Also make sure it isn't going to look funny, especially if you are thinking about an offset setup.