P1870 code 4L60-E

I have 1997 GMC K1500 w/5.7 engine.
For a long time I've had the shudder in OD going uphill but just last week I began having a very hard shift from 1st to 2nd. So I changed the fluid and
filter and the problem went away for a few days, now it's back. From the reading/searching I've been doing on the net it looks like I need to replace the valve with one from Sonnax which requires I remove and bore out the valve body to handle the bigger valve.
Quote from a mechanic: The 4L60E transmission also has a problem with a hard 1-2 shift when hot, and a 1870 code. When rebuilding the transmission, I recommend that you always replace the valve body plate and install a Sonnax Valve Kit TCC Regulator W/Sleeve. This is to help against having these problems in the future. On the 4L60E make sure you always us a AC Delco hardened shell when rebuilding the transmission.
Never having removed the valve body before, is this difficult?
Can I do this with the transmission in the truck?
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