High index lenses will not make your vision worse. High index lenses are made for people with strong prescriptions. If your optician makes your lenses in regular plastic, your lenses would be really thick because of your strong prescription. High index also called 1.60, 1.67, and newer technology has resulted in 1.71 and 1.74 is beneficial if you want your lenses to appear thinner. They look better and can even have BETTER optics than polycarbonate (a popular choice for children under 18 and those with medium-strength prescriptions). My opinion is that keep your glasses and stick with high-index lenses. Wear them for 1 to 2 weeks continuously so that your eyes can adjust to the new prescription and the new material. If after several weeks of continuous wear of your new glasses you feel like you are straining, go back to your optician and eye doctor and have them recheck your lenses.
It won't hurt your eyes, you're just not going to notice a huge difference in thickness till you go much above 3 and -3 .the thing with high index is needs to be sitting correctly on your face, because of the density of the material as opposed to other materials which are a little more leniant on where they can sit. Just like if you have a very high rx (like -9) they become much more sensetive to where they sit on your face. High index (or the higher the index you go) is also fairly reflective, and doesn't let as much light in as regular plastic or glass. You may want to go to an anti-reflective treatment sometime if you get a high index again, it would improve the clarity.
High index is neither bad nor good for the eyes, it simply makes the lense look better. Switching between glasses and contacts will not affect the eyes one way or the other. If you are comfortable wearing the glasses you don't need to wear them full time to adjust because, if comforrtable, you are adjusted. Wearing or not wearing glasses of any index will not make your eyes worse.
No, they can't do any damage to your eyes. They are a good idea for people who wear strong prescription because they make the edges of the lenses thinner but they are a rip off for people who wear weaker prescriptions.