This doesn't sound like a Magento problem, but rather, a general merchandising/accounting question.
Based on the information provided, I'll lay out a few scenarios, but keep in mind that there is no perfect formula for how to price your goods. You need to keep a wide variety of factors in mind to price products effectively and remember that some businesses change product prices multiple times per day. Changing product prices does not impact old orders/invoices.
1) Your 2020 goods are aging, and have a limited shelf-life. In this case, even though you've replenished your goods, you may want to put this SKU on sale temporarily to help clear out / close out these goods. This doesn't have to be a permanent price change.
2) These products are not identical, similar to how car models are not identical from year-to-year. You'll want to vary the SKU of the new products (ex. SKU= 12345-2021) and price them differently based on their costs/value.
3) These products are identical, and you simply need to account for the different costs. Thinking of your example, your costs have gone up. You could simply mark up the current price that you just paid your vendor, because that's the going rate. Chances are that your competition are increasing their prices as well. However, if you want to be more competitive, you could average your cost across the goods that you have.
Let's say that you strive for a 50% markup/margin. If you have 10 pieces that cost you $100 from 2020 and 5 pcs from 2021 at $200. You've spent an average of $150 each. So you could markup $150.
So, instead of marking up $100 to $150, or $200 to $300, you'd markup $150 to $225. That could drive in new customers and sales. If shoppers typically purchase other goods at the same time, this could be a good strategy for moving more merchandise, especially if you're still sitting on 2020 inventory.
Again, there's no perfect here, because this depends on supply and demand. If shoppers are willing to pay $300 for this item, you'll typically want to price your goods accordingly. Magento gives you the flexibility to change prices as often as you'd like, and at your discretion. Adjusting prices, running promotions like Catalog Price Rules, and otherwise changing costs charged to shoppers is normal, and will not impact your previous orders.
Best of luck!
