Going lithium

Welcome to the forum @War.

I really suggest starting with the basics:

What are your loads (Watt*Hours per day)--Assume you have done a lot of conservation (only loads you need, most efficient loads you can find, etc.)
Your location (or RV), regions you will be traveling too (hours of sun per day, seasons, WH per day of loads)
Other charging sources (genset, RV alternator, etc.)
Temperature of your "battery box"... Lead acid, AGM, etc. batteries work really well in sub freezing conditions. LiFePO4 (typical for home/RV use) batteries do better in hot conditions (more efficient, less self heating). However, they are several limited in discharge/charging below ~40F (near freezing). You are currently around Ontario Canada(?)... So low temperatures need to be addressed.
Solar array: You can mix/match panels to a degree... But you need to follow the rules... Typical rules:

• 24 volt bank @ MPPT charge controller: Suggest Vmp-array-std to be >1.3xVbatt charging or ~1.3x30vdc=39 volts minimum Vmp-array-std (standard test conditions)
• Adding panels in series... The Imp of the panels should be within 5% to 10% (i.e., 5 amp panel in series with 5.5 amp Imp max spread).
• Adding panels in parallel (or strings of panels): Vmp-array-std match within 5% - 10%. (i.e., 39 Vmp panel/string in parallel with 42.9 volts Vmp-array-std string(s))
• If mismatch >10%, then you can get into MPPT tracking issues, over current series panels, etc.)
• Respect Vpanel-max voltage for your inverter-charger: Panels, when they get cold, Vmp falls and Voc (voltage open circuit) rise... Very roughly:
• o Vmp-array-hot: 39.0 Vmp panel/array * 0.80 hot panel derating = 31
• .2 volts Vmp-array-hot (notice, just above ~30 volts charging for 24 volt battery bank).
• o Voc-array-cold: 31.2 volts Vmp-hot * 2x cold weather worst case way below zero temp = 62.4 volts Vmp-array-40 below zero

The above are rough planning rules for pretty much world wide (hot deserts to -40C arctic weather... If your location is a nice temperate Caribbean island--You do not have those temperature extremes.

Other things... Solar arrays are typically mounted at a tilt (or tracking) for homes/cabins... For RVs, the panels may be mounted flat to roof (not bad for southern USA, and closer to the equator)... However, the farther north/away from the equator, tilting panels can be a great help in winter harvests. Some folks have mounted tilting arrays on their RVs for this reason.

Other issuses:

Lithium Ion rechargeable batteries typically have a Battery Monitoring System. For  LIFePO4 batteries, a BMS just helps prevent battery damage. For other Li Ion chemistries, a BMS protects the battery bank and reduces the chance of fire. Deciding on a BMS and understanding how it may interface with your Inverter-Charger is important (some BMS can stop charging and discharging if battery bank/cells are over/under voltage or over/under temperature).

For you question--Need the Vmp/Imp and numbers of your various panels. Also min/max ambient temperatures, and Vpanel-max for your inverter charger (links are OK).

-Bill

If the 12 volt panels are good quality and test OK (i.e., Voc and Isc -- short circuit current in full sun) are OK--You can certainly use them.

With multiple panels, easy to test all under same sun conditions and look for "differences" (i.e., most panels are at 80% under your sun, and one panel at 40%--That panel is probably bad). Isc is mostly proportional to sunlight (i.e., 10% sun = 10% Isc).

Mixing old and new panels--It is always a mixed bag... You can have Vmp=36 volt large format panels (200 Watts or larger) and mix with 2x Vmp=18 volt panels in series for Vmp-array=36 volts (or 2x 36 volt panels in parallel with 4x 18 volt panels, etc.).

The other issue is that you can get 1x 300 Watt panel vs mounting 2x 150 Watt panels, extra wiring, different rack/rail spacing, etc...

Your old panels may have cost you $2-$5 per Watt, but the new panels can sometimes be found for less than $0.50 per Watt... Getting a "matched" set array may be a lot easier.

Issues with large panels--If shipping, the >~140 Watt panels usually have to ship by truck. Smaller panels can ship by UPS/etc.

Ordering 1x 300 Watt panel may cost more for shipping than the price of the panel (ordering a pallet load can save lots of money on shipping). Ordering a 140 Watt or so panel--Many come with free shipping these days.

Panels that are >~170 Watts usually need two people to safely move them around/mount on racks.

If you can get good pricing on large format panels (find overstock at a local solar installer, near a stocking warehouse, check shipping charges, etc.)... Get an array with one type of panels and use the old panels on a small backyard system, sell/give to a friend, etc.

-Bill