If you have Premier Support, you can contact them and have them add this filter for you. If you have a consultant that knows decent programming, hand the job off to them, it probably shouldn't take more than an hour or two to download the reports, apply some kind of script to update many files at once, then upload the results back.
Realistically, if this is just a one-off procedure, it's probably better to pay to have it done. If you must do it yourself, it's a pretty simple procedure. Install VS Code, the Salesforce Extension Pack, and SFDX, create a project, authenticate to your org, download the reports, use the Find In Files to find/replace in the new filter, and finally deploy the changes back to the server. It might still take an hour or two if you don't already have these tools, but someone who knows what they're doing could probably have this done in a few minutes.