May I just confirm the precise sequence of actions you propose:
1.Highlight all the images you wish to change
2 (Question) should this include as the first one one with the old time that needs changing?
3. Enter the new time against the image it has selected (not too sure why it picks as you described?)
4. Press Change all. NB at times it has shown only Change without the All which can explain why only the first one changes. A Selection problem in LR?
5. Wait a few minutes and check ( I have about 400 images selected)
The sequence is correct, but it sounds like you're not getting the selection right. How are you doing the multiple selection? There are basically two ways:
1. If all the images to be selected appear in sequence in the Grid (
and make sure you are doing this from the Grid, not in Loupe mode using the filmstrip to select): click on the first thumbnail in the sequence, then shift-click on the last thumbnail in the sequence. The cell border of all the selected images should now be a light grey, and the first one that you clicked on will be a lighter shade still (almost white). This is the "most selected" image, and it will be the current date/time of this "most selected" image that will be displayed when you select the Metadata>Edit Capture Time option.
2. The other way of doing a multiple selection is when the images to be adjusted are not in sequence in the grid (though if you sort on Capture Time they more than likely will be). But assuming they're not, click on the first image to be adjusted, then Cmd+Click on each of the other images to be adjusted. Again, the first one you click on will be the "most selected".
So before you commit the adjustment, verify that all the images you wish to change are correctly selected by looking at the thumbnails in the Grid...all should be light grey, with one having a lighter still appearance (it doesn't matter if the "most selected" image is not the first of the multiple selection, though most often it will be).
Once you have verified your selection is ready, then initiate the Edit Capture Time dialog, make the selection (option 1 or 2 can be used as you prefer), then click on "Change All". That should work