No, I wouldn't worry even a teeny but about your bag going out of style. But, yes, there have been some changes and variations. Here are the ones I'm aware of but some true Chanel experts / historians can weigh in with better information.
• Variations of material (caviar, lamb, python, velvet, etc.)
• Variations in quilting (diamond, chevron, straight line [I'm not sure what this quilting pattern is called] etc.)
• Single flap and double flap
• Interior matching the exterior color, a coordinating color or burgundy
• Additions of items to the chains or body of the bag (pom pons, ribbons, charms, etc.)
• Different style of chains (2 different bijoux, mademoiselle, etc. )
• Variations in the size of the double CC's used
• Sometimes the flap has been a little longer or shorter (reaching down further over the front of the bag or to a lesser extent)
And I think the flap shape and detail has varied. For example, the Diana classic flap's flap is different from today's classic flap's flap. (Someone clairify if the Diana is considered a regular classic flap or a different model of Chanel bag please)
But in every iteration the bag has still looked like a classic flap - no confusion on what the bag is at all. And from what I've seen every iteration is coveted and seen as timeless. Sometimes the bags that were only out for a year are some of the most desired. Now, I'm sure there is some limited edition that is universally disliked but I haven't heard of it.
So, IMHO, classic flaps never go out of style and never change enough to have your chosen bag be an outlier that looks dated. I think all of Chanel-dom would rain down curses of frogs on Karl if he (or his successor) tried to meaningfully change the classic flap.
The bags are amazing, "classic" and you can never go wrong with buying or owning one.