I, personally, just did 4 panels yesterday which probably equaled your entire sign. Mine were 24" x 44" and your's looks to be about a 4' x 8' double sided. If you're only doing the top, it looks about 22" x 96". It took me about 1/2 an hour to do all four of my panels and that includes cleaning the glue off. The panels were about 10 years old.
I use a regular double edged razor blade to start. Got it all off in minutes. After finishing up all four, put some 91% alcohol on it. Let it sit about 15 seconds and scraped a little more with the razor blade. After it was rather clean, I wiped it down with a clean paper towel and 91% again and they were ready to letter in 1/2 hour. Lettered them up and the customer picked them up around 4:30.
It's not hard to re-use the old panels over. If I were to guess, with the economy the way it is..... most people are trying to re-use stock over again. If ya can't do it and do it in a reasonable amount of time, like most here are complaining, then ya might hafta start with a new substrate. As for ghosting, your new copy will probably hide it for the most part. People will be reading fresh vinyl that will stand out at 100% where the ghosting is maybe 5% at best. The new lettering will overpower that without a problem. Be finicky and make sh!t up with your customer and you can kiss the job good-bye, altogether.