when you revalue an asset arent you going against the cost principle when writing its current value on the b/s

Answer 1

The historical cost principle is a necessary basis for accounting; otherwise since costs of things move all the time from inflation and price changes, the accountant will be spending all his time changing the value of everything in his accounts. However as you have noted, there are exceptions to this rule. PPE can be recorded subsequent to initial recognition using either the cost or revalution model but there are criteria to be fulfilled. The fair or market value of marketable securities needs to be disclosed on the balance sheet or in the notes to the financial statements. Thus, accounting for marketable securities represents an exception to the cost principle too. Many of the FASB’s decisions are built on the principle that a business combination should result in the recognition of the acquired business at its fair value. This principle is different from the traditional cost-based foundation for business combinations.
However, it is consistent with the FASB’s ongoing efforts toward recognition of fair value amounts in financial statements. I don't know where you are in your accounting studies. Suffice to know that there are allowed exceptions and don't let that worry you too much.

Answer 2

You are now reporting replacement value. But it depends on the law of the country if this is allowed.