When a fully-built Masonry and Spyglass Overlook tower are placed so that their ranges overlap, placed Arrow and Scatter towers will remain as wood and won't convert to stone.

When the Spyglass Overlook tower is removed, placed Arrow and Scatter towers don't retroactively turn to stone.

The towers will only turn to stone when a new Arrow and Scatter tower is placed with no Spyglass Overlook tower in range.

I think this is because Spyglass Overlook towers converts Arrow and Scatter towers into a different (code-wise) tower that hasn't been coded to turn to stone.