Need more information - when was the last (most recent) transmission service performed? How many miles since the last fluid and filter change? If over say 36k miles or 3 years time - then time to change the fluid and filter anyway - the additional 8k miles will not matter much in this case.

The rule that I follow here is if the tranny oil is still mostly red in color as opposed to dark brown or even black, then it is Ok to get it changed. If it is very ugly dark or black, you may make slipping happen that isn't right now because of all the abrasive contaminant in the oil if you change it.

Yes, replace the tranny. The theory here is changing the tranny oil did not cause the damage to the tranny to make it slip (if it slips at all) but it simply exposes the amount of wear that there was already on the clutch packs within the unit. The damage was already there, you just speeded up the process by removing much of the contaminant that was kind of the "glue" that was allowing transmission function without noticing any slippage. A healthy transmission should not need any help with contaminated oil to prevent it from slipping.Its supposed to operate normally with nice clean uncontaminated red tranny oil of the proper specification.

The chain lube centers like Jiffy Lube for in stance have the machine that changes out all the trans oil without removing the trans oil pan. The cost is about $175 or less, keep in mind that a 12 quart case (the service will use at least 10 quarts) of Honda trans oil costs about $8.00 a quart. A rebuilt tranny is gonna cost about $2500 about for the full job with at least a one year 12000 warranty, maybe two year 24000.