In v2.4 the command line option -f now accepts frame ranges and frame lists. That was introduced very early in the 2.4 alpha, quite a while ago. Nothing should have changed when this was publicly released. What version were you using before you upgraded? What command are you passing to Terragen to render the packets?
From the command line documentation:
-f <frameRange|commaSeparatedFrameRanges>
Set the frame range or list of frame ranges to render when -r is also used. If -r is not used, the current frame is set to the first in the range.
The list can contain any combination of frames or frame ranges separated by commas without any whitespace.
Example: -f 34
Set the frame to render to 34.
Example: -f 1-100
Set the frame range to render from 1 to 100 (inclusive).
Example: -f 1-100/5
Set the frame range to render from 1 to 100 with a step size of 5. This produces the sequence 1, 6, 11, 16, ... , 96.
Note that frame 100 is not rendered because 96 + 5 = 101 which is outside the specified range.
Example: -f 34,37,38
Set the list of frames to render to 34, 37, 38
Example: -f 1-50,66,71-100/10
Set the list of frames to render from 1 to 50 (inclusive) followed by 66 followed by 71, 81, 91.
Note that frame 100 is not rendered because 91 + 10 = 101 which is outside the specified range.
Matt