What is the guideline on change in job description when changing a job on the basis of approved I140 from my previous employer?

You may be confused here. Did you act upon the approved I-140? More information is necessary.

I lost an unopened letter that i got from US immigration office . Is there any way to know what that letter contained?

Or, simply call the USCIS 1-800 number and request that the letter be re-send to you. Also, verify that USCIS has your current mailing address.

Visitor visa

Congratulations! Yes, you can provide a "letter of support." However, your mother-in-law will need to prove her nonimmigrant intent. An experienced...

I concur with the above answers. Consult an immigration attorney!

Can I file re-entry permit then leave US without doing biometrics. Can I return to do biometrics later within one year?

No. Stay here until the biometrics are completed. Otherwise you will be flying back and forth multiple times and wasting money! Discuss your travel...

Can I attend a private university on an L2 visa this fall if my visa expiration is July 2018?

Yes, but only until July 2018, unless you extend or change your nonimmigrant status. Is the principal L extending status? If yes, then why switch...

How long can I stay in the country if I quit my job (which sponsors my H1B work visa)

Remain at the job until you secure another. Otherwise you are limited to 60 days and may have to leave the country.

Most likely, either an incorrect filing fee or wrong form was submitted to USCIS. What did the employer's attorney say?

Change of jurisdiction of I-140 and I-485

More importantly, make sure USCIS has your updated address and contact info.

H1 B/L1 visa cool off period eligibility criteria

Yes, you must be outside of the U.S. for 365 or more.