Can I change the base currency of my Landing Account?

No. The Landing account is specific per currency.  You cannot change it but you can simply create New Landing account in any available currency (USD/EUR/GBP/RUB/PLN)

https://account.ampglobal.com/MyAccount/Index#

and then submit request for currency conversion (Transfers / Internal Transfer) or simply fund your New Landing Account with your desired currency.

https://account.ampglobal.com/FundTransfer/FundAccount 

See: Documents section/ Broker Forms for conversion fee.

You will be able to create MT5 Trading Account as well in new currency simply by selecting e.g..