Simply opening a v8.5 file in v10 will not break any functionality.
The fields you are seeing on the layout are merge fields. Do you have another layout that displays actual fields where you can do data entry? If so, is there anything in those fields?
I realise that you are a novice but what do you mean when you say "Jobsheets are created from Client Database and Invoices from Jobsheets"? Do you know if there is a relationship between the tables?
Hard to know what you are dealing with there.