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

[Alex Kachanov]

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!