Let's assume your current amp is loud enough and has the features you require to get the job done. Whether a different amp will be an upgrade is totally dependent on your subjective perceptions.
Let's use the Mesa 800W subway amps as an example. There are four models. D800, D800+, WD800, and TT800. AFAIK these amp all use the same output section, but the way the outputs are tuned varies a bit across the product line. Each of the amps is rated 400W at 8 ohms and 800W at 4 or 2 ohms. So you get pretty much the same level of power regardless of which amp you choose.
The variance in the way the output sections are tuned means they respond a bit different as the power is increased. Also, AFAIK the TT's output section is tuned to interact a bit more with the impedance of your speakers than the others. I believe this is intended to give it more of a tubey quality.
The D800 and D800+ preamps are pretty similar. The + has a few more features, including a bright switch and a variable HPF. AFAIK otherwise they sound and respond fairly similar. Both of these amps are pure solid state.
The WD800 and TT800 have fairly different preamp architecture and voicing. Also these amps are hybrid with tubes in the preamp. The TT800 has a two channel preamp. The Boogie channel is based on the old 400 and 400+ all tube amps. The Subway channel is based on based on the D800+ preamp, but uses tubes.
Which Subway 800 amp is best? Well that's for you to decide.