An important thing to remember now that you switched to Lightroom is that you should try to do as much from there as possible. You will get used to it in no time. As you just found out, LR maintains a lot of information in its database and modifying the files on disc will put you out of sync. Another hurdle is to resist the urge to move files around from Windows Explorer; make sure you do it from Lightroom or you will soon spend some quality time telling LR where to find your old files.

Anyway, to solve your problem, select the files that are out of sync, right click and pick "Metadata -> Read Metadata from file" from the context menu or the main menu. I'm pretty sure "Synchronize Folder" works if the "Scan for metadata updates" option is ON, but this would also import new images and you might not want that.

This goes both ways of course. If you update some Metadata in LR and want to make sure it is accessible to third party apps, select your files and use "Save Metadata to File". You are probably wondering if this can be done automatically for you, and the answer is yes, by checking the "Edit-> Catalog Settings-> Metadata -> Automatically write changes into XMP" option if I recall but I would advise against it for performance reasons.

**added 7/18/13, using LR 4.4: this method does not work if you have chosen to have Lightroom export its metadata into .xmp "sidecars" (Catalog Settings->Metadata->Automatically write changes to xmp) or otherwise have .xmp files alongside RAW or similar purportedly read-only images UNLESS the tool you have used external to LR (ExifChanger, for example) has written a line for DateTimeOriginal or DateTimeDigitized into the .xmp file. ExifChanger for one does not, so the timestamp in the .xmp file never reflects the change made, but it is shown in the file's own exif data, which LR seems to ignore.