A solution which will depend on Class voip always coming directly after the first priority:

use strict;
use warnings;

while (<DATA>) {
    if (/^Class voip/ .. /^\s*$/) {
        s/priority/bandwidth/;
    }
    print;
}


__DATA__
Class control
priority 5
Class voip
priority 30
Class video
priority 40


Class control
priority 10
Class voip
priority 25
Class video
priority 45
[download]

Output:

Class control
priority 5
Class voip
bandwidth 30
Class video
bandwidth 40


Class control
priority 10
Class voip
bandwidth 25
Class video
bandwidth 45
[download]