I'm confused too.
Usually you have Decimal (0-9) or HEX inputs (0-F)
BCD outputs in either complement or non-complement form
I have NO IDEA what decimal output is. It could be a 1 of 10 line. e.g.
You have to get the datasheet. Take a look at
www.farnell.com or
www.newark.com or even
www.digikey.com for stuff with datasheets.
There is also different ways to activate a thumbwheel switch:
1) A lever - it just moves through the positions and is very rapid
2) A push button for increment and 1 for decrement - Not as fast as #1
3) A flip type motion. Generally used for infrequent changes
Now, you might be able to convince the powers that be to use an encoder: See
https://www.sparkfun.com/products/10596?
So, if you had a display, you could select the digits and increment/decrement them.
This is no more than a suggestion.