What's the difference between complete tear out and retrofit?

Answer:

COMPLETE TEAR OUT refers to removing everything down to the studs starting from scratch

• BENEFIT: the better method of installation and maintain viewing area. 
• DOWNFALL: more product, more materials, more labour = more money

RETROFIT refers to removal of operating sashes, inserting a window into existing window frames

• BENEFIT: less money. 
• DOWNFALL: any air filtration behind an old frame cannot be eliminated.