Steve Smith is right. You should never have to change the scale of a drawing. It should always be drawn at 1=1. You might change the scale or size of your title block around your drawing so that it will fit on a particular size of paper when you plot it at 3/8"=1" or 1/4"=1", but you never want to scale your drawing up or down. If you do that, you will never be able to measure any distances on your drawing.
This will also make it easy to insert blocks, because you never have to worry about what scale to insert your blocks. Unless you tell ACAD otherwise, it will insert them at 1=1.
If you have any questions, feel free to email me at:
dmartin@carrollconsultants.com
Danny Martin