diff-patch: apply .diff created for old file version

Hi!
Trying to apply some patches in unified diff format to mozilla source.
But these patches where created some time ago and source code where they are 
applied already changed.
Example:
.diff patch is:
---
aaa
bbb
ccc
+ccc2
ddd
eee
---

And nowadays source file looks like
---
aaa
bbb
bbb2
ccc
ddd
eee
---

so patch tool says "hunk FAILED".
Tried manually changing diff files, but nothing helps. How should I change 
them? How to change offset numbers in
@@ -a,b +c,d @@
lines?

Thanks!