The age old question.
Do you rework/polish the old website or get a new one?
Here are 5 reasons to keep your old website:
#5 - If you have no intention of updating your websites content
#4 - If you update your websites content regularly but lack the budget for a new site. Fresh relevant, regularly updated content can be as valuable as a new website.
#3 - In 20 years your old website might be cool again.
#2 - If your google rang is #1 on all your keywords. Don't mess with a good thing right?
#1 - If you have a 10% conversion rate, meaning 10% of website visitors become customers, ie generate leads or sales.