Most times you will have to remove a portion of the siding, but sometimes it depends upon various factors. How large is the new window? Same size or larger or smaller? Check with a professional to get the best information regarding your specific situation.
I have installed hundreds of replacement windows from the inside. They take the place of the old sashes and slide into the original frame.
We had a company measuring our windows for replacements and they asked if we just wanted inserts or the whole window replaced. We wanted the whole thing done. But if you measure within the casement you might not have to remove siding. It will mean you window will be a bit smaller. Another thing to consider, look carefully at your old windows. When you frame in the new window could the new trim overlap the siding? That would mean any adjustments made to the siding could possibly be covered by new trim.
We have replaced from inside and outside and each required removing the sill and trim but not disturbing the exterior siding or interior wall. The window size remained the same. If trim is solid and removed carefully, it can sometimes be reused.