There are no proposals at this time. It requires Congress to pass a law, and have it signed by the President to change faces of the currency. The US Treasury may have some researchers doing a study on the subject, but they cannot do this without Congress' approval. As far as I can research, there is no indication of a willingness of Congress to change the portraits on the currency. The updates on the security measures of the US Treasury incorporated into the paper and printing (to prevent counterfeiting with modern computers, etc.) did not change the faces on the denominations, just adjusted their size and put them slightly off-center to make them easier to identify. The reverse of the currency was changed, and different figures were added to the bills and colors. But no change of portraits.

Coins:

This does not apply to the Presidential Set series of $1 coins, which are scheduled to bear the busts of all the deceased US presidents eventually, according to a law passed a couple of years ago. (An ex-president must be dead at least for two years before the US Mint can issue a coin with his portrait on it (unless Congress decrees otherwise, as they did after the Kennedy assassination.)

Proposals:

There were some proposals to replace Hamilton on the $10 bill with Ronald Reagan, but no bill passed the committees of Congress before the end of the last Congress. There was a lot of vociferous opposition from many people. It probably will not be politically possible to pass that bill in a Democratically-controlled Congress any time soon.