Website purchases are assets or expenses?

I've been operating as a sole proprietor.  I usually buy a few websites each year for their income.  I fix them up, improve them, and hopefully increase the income or create income when there was none before.  I typically hold them for more than a year. I've been treating these purchases as expenses and deduct the entire thing in the tax year I bought them.  I just realized I probably should treat them like assets and not expenses, any opinions on that? And if so, should I amend all my previous tax returns or start anew?