Not unusual to change position when you have been riding longer
It is not unusual for your position needs to change on a bike. It sounds like you have lost a bunch of weight and are probably more flexible so you may need to have a longer reach now.
Be careful how you achieve the longer reach, however. If you move just your seat back, you may find that your knee is no longer over your pedal spindle and you could cause problems that way. Go to the LBS, have them move your seat some and measure with a plumb bob, how your knee is over your pedal spindle. You may need to get a different stem, get a different stem and move your seat back a bit? It is hard to tell without seeing the fit.
I've been shortening and raising my stem and handlebars as I get older because I have a bad back. Also, probably just because I'm getting older. I also am not as flexible and wear more clothes in the winter so I will lower my saddle in the winter. I raise it back up in the Spring as I put more miles in and wear less clothes. Fit can be a slightly variable thing. You just don't want to change things like saddle height in big jumps. Just go up or down in small increments and you will be fine!
When do you feel the need to scoot?
When do you feel the need to scoot back on your saddle?
Remember when you are climbing and you scoot back on the seat a little bit, it gives you more POWER! So, if you are scooting when you need more power that is a natural and good thing!