increment existing numbers by 1

I found this. I think it is what I want but I don't understand what it
is doing or how to initiate it.

http://vim.wikia.com/wiki/Substitute_with_incrementing_numbers

Incrementing selected numbers

function! Incr()
let a = line('.') - line("'<")
let c = virtcol("'<")
if a > 0
execute 'normal! '.c.'|'.a."\<C-a>"
endif
normal `<
endfunction
vnoremap <C-a> :call Incr()<CR>


I want to increment numbers that exist by 1. I also don't want it to
change other numbers only numbers in a certain pattern. The pattern
that I want to manipulate is:

:'y,'zs@^[ ]*<navPoint id="navPoint-\zs\(\d\d\d\)\ze">$@not sure what
I would put here@

I have tried several different attempts with s@pattern@replace@ but
each one would skip every other pattern.

So if the number was 242 I want it to change to 243 and then that one
change to 244 etc. That would happen on only the pattern in the range
'y,'z or the actual line numbers could be the range. The pattern
number of each line needs to become the pattern number of the next
pattern after it. The spacing between the lines with the pattern
varies from five lines to six lines.

The number in the pattern is the only number on the line.

I have several hundred lines scattered over about 3000 lines that need
to be edited but the only way I know how to do it is manually one line
at a time. I am obviously trying to avoid that pain.

Can anyone point me to a solution or tell me if I found the solution
and how to use it?

Thanks for any help

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