After 30 days, the Initial and Maintenance margin requirements for US mutual funds is 25% of the Net Asset Value of the fund. End of day Reg-T margin is set to 50% of the Net Asset Value. When you transfer a US Mutual Fund from another broker into your IBKR account, the Mutual Fund position arrives with the original acquisition date. In a margin account, the IBKR credit system will calculate the 30 day period from your acquisition date and your Mutual Fund's margin requirement will automatically be adjusted on the 31st day. Please note that while a mutual fund is non-marginable and has a 100% margin requirement for the first 30 days, the mutual fund position also has a 100% charge to SMA in a Reg T account for the first 30 Days.
After your broker has sent the information to IBKR, you can see the Fund's acquisition date by creating an Activity Statement with "Lot Details" and clicking on the "+" sign next to the symbol in the "Open Positions" section. The position transfer date can be found in the Activity Statement's "Transfers" section on the day the transfer was received by IBKR.