Currently have an architectural drawing with the scale of 1" = 100'
Need to convert the scale to 1" = 40'
Can I have the current drawing enlarged to accomodate this.  
If yes, how much do I need to enlarge the current drawing?
My non-mathmatical mind keeps whispering 160%.
Please tell me your professional opinion.

Close!

At the current scale, 1 inch = 100 feet.

At the new scale, you would need 2.5 inches to represent 100 feet.

So you have increase the scale by 150%.

The arithmetic looks like this:  (2.5 - 1)/1 = 1.5 x 100 = 150%

(Note, however, that the language of percents can be confusing.  A
drawing that is 100% of the original is the *same size* as the
original, but a drawing that is 100% *larger* than the original is
double the size -- 200% of the original.  Similarly, we can say that
your new scale is 150% larger, which gives you a picture scale that is
2.5 times  -- 250% -- larger than the original scale).

Have I cleared things up...?  Or only confused you further?  

If the latter, please post a Request for Clarification to let me know.
 I'll be glad to keep working with you until it's all crystal clear.

By the way, if you are working with a professional printer, they
should certainly be able to scale up the picture for you if you just
give them the new scale that you want for the drawing.

One last note:  I used to do a fair amount of drafting myself, and I
remember clients who certainly had an expectation that an enlarged
scale was going to provide more detail than the original drawing.  If
you are working with others who will use this drawing, it pays to be
clear about what their expectations may be for the enlargement.

A drawing that is 100% larger would actually be quadruple the size, as
the size (area) increases as the square of the linear measurement.

This is why enlarging from A4 to A3 paper doubles the area of the
drawing, but isn't 200% of the original in scale.

Owain

Thanks Owain for adding that piece of information.  I was careful to
keep my answer focused on the linear scale, while avoiding discussing
the effects on the area, as I thought that would only confuse things. 
But if Bigbill-ga would like any additional information on this topic,
I'd be happy to provide it.