I could darn near write a book with my opinions on the subject, but I'll try not to. My opinions do however come from 27 years of experimenting & testing bows with fast flight strings, with this subject being just one of the parameters of the testing.
I don't/won't encourage anyone to use a FF string on an older bow, but I'm not going to say it won't work either.
I've been doing it for years, and only had one break. The one that broke, broke when my daughter dry fired it, so I'll never know if it was the string, or just the old bows time to die. The FF string didn't have padded loops (this was many years ago). I have to wonder, if the loops had been properly padded , would the bow have survived the dry fire? I suspicion it would have.
I have a friend who broke an old non FF rated bow with a FF string on it. He was shooting a 60# bow, but only shooting an arrow that weighed 350gr. He blamed it on the string. I suspect the light weight arrow had more to do with the breakage than did the string.
As someone mentioned above - the glues used in the lay up of the bow is a huge determining factor. If the bow is a pre compound era bow, then you might have cause for some concern in that area. If the bow was made post compound, the it is likely it was glued up using the better glues.
Also as mentioned above - The string grooves, and how well they are made (or not) is also a huge factor. String grooves that force the string to place heavy pinpoint pressure at the shoulders of the grooves are not ideal. String grooves that more evenly distribute the pressure completely around the limb tip are the best. That applies to any bow whether it is FF capable, or not.
Properly padding the loops make a big difference. Padding the loops helps the string to evenly distribute the afore mentioned pressure points, which increases the surface contact areas between the limbs & the string, and increases the structural integrity/durability of both.
Notice I said PROPERLY PADDING , because there are ways to do it that work, and ways that don't work so well.
If you decide to do it , I suggest you forget about speed, and concentrate more on arrow weight. You'll likely be able to shoot a heavier arrow with a FF string at the same trajectory as a lighter arrow with the dacron string. The heavier arrow will help take some of the stress off the bow, plus a heavier arrow shot at the same trajectory you are used to is a good thing in my opinion.
A lot of old bows that have broken during the use of FF strings, were not the fault of the string, but were related to other things, and the string was just an easy avenue of blame .
There are a lot more old bows out there with FF string on them, and getting away with it, then there are those which have broken.
Here's a story for you:
"I was running a 3D shoot about 20 or so years ago, and was having a lull in the action for a bit. As I was resting, I watched a fella pull into the parking lot, and start getting his gear out of his truck. He walked around to the passenger side of the truck, and got his stuff out from that side. When he closed the door of the truck, he slammed it on the bottom limb of his bow. I mean slammed the door shut on the limb tip. He stood there, cussed a blue streak for a bit, and then strung his bow, and inspected it. Must have looked ok to him, because he grabbed his stuff, came & signed in, and then headed over to the practice line where his buddies were waiting. They all stood the shooting for a bit, and then headed out on the 3D courses. About lunch time the group came walking in. The guy who slammed his limb in the door was walking along, and again cussing a blue streak. The bottom limb tip on his bow was broke almost completely off. They all ordered some lunch, and sat down at a pick nick table not 15 feet from where I was to eat their meal. The conversation was heavy, and very colorful between them. The guy with the broken bow was !@#%ing Fast Flight strings this, and *&^%$ing Fast Flight strings that, and going on, and on, and on about it. I sat there for about 15 minutes listening to the whole thing, and finally I just couldn't take it anymore. I spoke up, and said Hey fella. You don't think slamming the door of your truck on that limb had anything to do with it breaking? At that point all of his buddies just looked at him with amazement, and laughed. He gathered his stuff, and left the scene in a hurry."
The point to this story (very true by the way) is this - Fast Flight strings when in use on old bows that break, get the blame whether it was their fault or not. We'll never know the real numbers of broken bows due to FF strings, but my experience leads me to believe that the number is pretty low compared to the number of those broken for other reasons, and the string just got them blame.
Bottom line is - YES, there is a risk , but not nearly as dire a risk as some would lead you to believe, and with a little common sense & care on your part, you can likely do it, and survive it,
BUT
the decision has to be yours.
Here's how I look at it from my perspective - I know there is a risk, and if I have an old bow that has any significant vale to it (sentimental or monetary), then I don't use a FF string on it if it isn't FF capable. Then again, with an old bow like that, I probably won't shoot it anyway. 8^)
Rick