The Upsize Request: 2.5 Tons to 3 Tons

No - not until you have a load calculation.

Reasoning:

• 'Never kept up in July' is a symptom, not a sizing verdict. On a 1978 house with leaky R-4 attic ducts and one undersized return, the most likely cause is distribution loss and envelope load, not insufficient nominal capacity. A 3-ton unit pushing into the same leaky ducts will deliver a similar amount of cool air to the rooms while drawing more amps.
• Oversizing penalties: shorter run times, poor latent (humidity) removal, wider temperature swings, higher airflow and noise, and a coil that runs cold and wet. In a humid climate that means a clammy house that still feels warm.
• Matching: the new condenser must be paired with an AHRI-matched indoor coil/air handler. Bolting a new 3-ton condenser to a 20-year-old dirty 2.5-ton coil loses the rating, may lose the warranty, and can violate the mechanical code and utility rebate requirements.

What you propose instead: a Manual J load calculation using actual window areas, insulation, and infiltration; a Manual S selection from the resulting load; and a scope that includes duct sealing and return upsizing. Show the homeowner that this is what actually fixes 'it never kept up.'

Size: a 3/8 in. liquid line with a 3/4 in. vapor line at 35 ft is generally within the manufacturer's published line-set table for a 3-ton R-410A system, but you must confirm it against the specific manufacturer's table (which gives maximum length, maximum vertical separation, and the capacity penalty per foot) rather than assuming. Check that total equivalent length, including fittings, stays under the limit and that oil return is adequate at the lowest load - no traps are normally needed on horizontal runs, but a vertical riser may need one.

Cleanliness and compatibility - this is the bigger issue:

• The old system used mineral oil; the new R-410A system uses POE, which is hygroscopic and will not tolerate mineral-oil residue or moisture. Either replace the line set or flush it with an approved flushing agent and blow it out with dry nitrogen.
• Install a new liquid-line filter drier - never reuse an old one, and never install a suction-line drier as a substitute for a clean system.
• Flow nitrogen while brazing every joint, and use a heat-sink or wet rag at the service valves.
• Pressure-test with dry nitrogen, then evacuate to 500 microns or below with a decay test before releasing the charge.
• Verify final charge by subcooling per the manufacturer and confirm airflow near 400 cfm/ton after the duct work is addressed.

Electrical: a disconnecting means within sight of and readily accessible from the unit; correct conductor and overcurrent sizing per the nameplate MCA/MOP; proper grounding and conduit; condensate pump/float switch wired so a plugged drain shuts the unit down.

Condensate: correctly sized primary drain with slope, a trap on the negative-pressure side of the coil, secondary pan with a safety switch, and no drain termination where it can cause damage.

Clearances and placement: manufacturer's minimum clearances to walls, fences, and the building; unit level on a pad; service access for coil cleaning; refrigerant piping protected from physical damage.

Code and permits: mechanical permit and inspection where required; equipment listed and installed per the manufacturer's instructions (which the code adopts by reference); combustion air and venting untouched; condensate and refrigerant lines sealed at penetrations.

Commissioning: verified airflow, superheat/subcooling, temperature split, amp draw, and thermostat operation - recorded on the invoice.

Commonly skipped: the permit and inspection, duct sealing after the equipment change, verifying MOP against the actual breaker, and documenting the final charge and airflow. Those are exactly the items that turn into callbacks.