Prichard,
Ideally best approach would be use EMIGALL migration object INSTLNCHA. (You can copy from SAP Migration Company into your Utility Migration Company and make adjustments based on your requirements).
LSMW will work seamlessly for simple "changes" or "break-fixes" to Premises/master data objects, etc., because you're updating master data/fields, going through simple one-off screens, without any time slices as such. For seamless updates to a little more complexed objects (including time-sliced data/screens), it is best recommended to use Standard EMIGALL objects (if available).
Having said that, have you tried executing the LSMW in foreground mode (Not - Errors Only Mode)? Do you see the data values populated into the field fields Instln, Billing Class, Rate Category? And what happens when you click SAVE? Check your OK Code values in the recording and compare to what LSMW attributes show.
Also, please check the data accuracy in your file: 1) ensure values are correct; 2) If incoming values are the same on the existing Instln (compared to values you are trying to change), then perhaps, when you hit SAVE, "no changes are made". Therefore, LSMW may show a success without any updates as such. So please check on your data values too.
Regards,
Ashvinder
