Combining 2 Sheets Merging Based on Unique Identifier

Ok, I have to admit I'm at wits end here. I pull data monthly (usually 2,400+ lines). Let's start with the file "Aug Save". Columns A - G could potentially change. Columns H remains blank. Columns I - O remain the same. It's historical to see previous changes month to month for that particular investigation.

"Sept Save" is the latest pull. Invest ID is a unique identifier. New investigation = new number. Columns A - G could potentially change. New supervisor, investigator. etc.

Additionally, there will be many NEW investigations not on previous sheet. So...I have to manually verify line by line and match up new data w/ old.

Is there a way to merge the two sheets based on the Invest ID. Overwrite Columns A - G (in this case, use "Sept Save" data, merge into "Aug Save". Columns I - O would remain untouched.

As far as new data, I could easily grab investigations with "OPEN" that are 30 days are under. I hope this makes sense. This report is monthly and is time consuming at the moment.

Lastly, I only have data in "Sept Save" to column G as the other columns will not change. Do I have to have identical columns in both files in order to get what I need?

Thank you