Unless specified in the original contracts of that 6-year body of work, you are not obliged to keep the original working files of past projects. That you do so is only for personal archiving. Shenanigans like this are the exact reason for that, and I advise you to include a statement to that effect in your terms and conditions from now on.
Try and explain this client how valuable source files are. Also explain to them that they are probably messy, as you were most likely not expecting to have someone besides yourself working on them.
Explain to them the strain that this request puts on your relationship. Explain that source files are the core of your work for them, and that handing them over is like giving away your trade secrets.
Tell them you understand they want to cut costs and response times by having an on-site designer. Are you too expensive? Aren't you efficient enough? Ask them these questions. If they say yes, offer to renegotiate prices or maybe even offer to work on-site for a few hours per month. Make them understand how much this means to you and that they are essentially telling you that your work isn't good enough.
If they persist in their intention after that, do stress that the procedure they are asking you to undertake includes the transfer of copyright (the 300%) in addition to a hefty hourly fee for preparing the files to be handled by someone besides yourself.
If that doesnt scare them off, create a quote for them that includes 300% of the original fees of each and every project you are handing files over from. Adding the hourly fee, this quote might approach two years worth of wages or something similar. That's intentional and is meant to scare them off. You might want to overestimate the number of hours you expect to spend on prepping the files. Make them feel you aren't letting go easily.
That quote is essentially you telling them, 'do you really want to do this?'. I would be offended to be cast aside like this to say the very least, and you should make them feel that.
This answer is hinging on a couple suppositions: one, that you have effective contracts for all the work you did, including terms that allow you to retain the work's copyright upon (original) delivery. And two: That you realise that actually sending the quote will most probably destroy your relationship with this customer in one fell swoop. If at all possible, I would try and explain them what they are doing: casting you aside.