Saline for 18 years then silicone, what should I expect to have to do to fix them? (Photo)

Hello,
Looking at your picture, it appears that your left implant has "bottomed out", and is sitting lower than the right based on the nipple position and where the scar is sitting. This could be improved with either tacking the pocket to push the implant into the correct position, or doing this and adding mesh for reinforcement.
Regarding your silicone implants, it is surprising to hear someone say that they feel harder than saline, often saline is noted to feel harder and people feel like silicone feels more natural. This may have been because your silicone implants are a thicker gel and your saline implants were underfilled, making the silicone feel harder and the saline feel softer, but this is hard to know. The other thing that may be going on is that you may have some degree of capsular contracture, meaning the scar tissue around your implants has thickened and is causing them to feel harder and change shape a bit.
I wouldn't say that there is anything wrong with swapping back to saline implants if that's what you wanted to do, but I wouldn't do so assuming they will feel softer, I think there are some other issues at play here. But atleast this information might give you a better idea of what might be going on so you can make the best decision with your future surgeon.
Hope this helps!