I have Raspberry Pi 3 B+,and I'm considering upgrading to a Raspberry Pi 400. I understand that it is recommended to use a 5.1V 3A power supply, but can I use the 5V 2.5A supply I already have?
Yes - you can power your Pi 400 with a 2.5A supply, but you will be limited on how much current your USB devices can draw. There's a table in the "official" documentation that lists the Typical Power Requirements for all RPi models.
As you can see, 3.0 amps are recommended for a power supply to support the RPi 400, and this figure includes 1.2A for USB peripherals, and 0.8A for bare board. Since the RPi 3B+ supply is rated at only 2.5A, you should plan on reducing the USB peripheral load on your 400 by 0.5A (to 0.7A) to maintain the same "margin."
Be sure to read the manufacturer's notes beneath the table; in particular the final sentence
If you’re not sure [of the power rating of the devices you plan to connect to the Raspberry Pi], we would advise you to buy a powered USB hub. In this respect, your situation is no different than if you had a 3.0 Amp power supply: This table, and the accompanying notes, provide guidelines to help you manage your power budget; the only difference really is that your power budget with a 2.5A supply is a bit smaller. You can overload a 3.0 Amp supply just as you can a 2.5 Amp supply.
You should understand one likely consequence of drawing too much power from your supply is that its output voltage will drop below the low voltage threshold of 4.63V. If you're concerned about breaching this threshold, you should keep a watch for the "lightning bolt" display on the monitor, or some of the other low voltage indicators.
If you want to go a bit further, you may also wish to read the documentation on
vcgencmd; in particular the
get_throttled command which will provide a bit pattern (binary) value indicating the occurrence (past or present) of low voltage conditions. And finally, if you're interested in how the RPi detects a low voltage condition, you can read the analysis in this answer.