You can change GK to KK. Mandatory registration & tax fees is about 100,000 yen, but this does not include professionals, such as lawyer’s fees.
Governance structures are very different between KK an GK.
KK is a stock company, so each shareholder has right to control the company according to the number of share. For example, the shareholder who owns the most share has the strongest power regarding the company’s decisions. Also, the shareholders receive the dividends from the company according to their shares.
On the other hand, the governance of GK is more flexible compare to KK. Governance of GK is totally depend on the Article of Incorporation. For example, you can delegate all authorization of the company to one person if you determine in the Article of Incorporation