Amend immediately. You have a continuing duty to ensure that the schedules are correct. Amendments can affect the rest of your case depending on what is being amended. Adding creditors in a Chapter 7 is not that big of an issue if done quickly to provide them adequate notice and time to file any objections. Adding property may be an issue if you cannot exempt it all.
Amendments must be filed with the court and served on affect creditors. Local rules govern as well. You may want to seek counsel if you are not currently represented. Bankruptcy is a complex, paperwork intensive process.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]
It is best to amend promptly. If you are represented by counsel consult with them promptly and document your request that he or she amend. If you are pro se I suggest that if your bankruptcy court has a help desk or can refer you to legal aid that you talk to someone who has experience in bankruptcy cases. If the problem is a failure to properly list assets this can have ramifications on your discharge so if this is the problem it should be corrected asap with the assistance of counsel or by the means suggested above. The amendment would have to be filed at the courthouse and served upon those parties affected. Local rules also need to be consulted. Consult with counsel or the other sources suggested and it also might be advisable(assuming the problem was an innocent and inadvertent mistake) to make some disclosure to the Trustee concerning the inadvertent mistake, if, by any chance, it has not been remedied prior to the creditors meeting.
This response is not intended to create an attorney-client relationship, nor is it intended to be legal advice. This response is merely for educational/informational purposes and is and is not meant to be a legal opinion or legal advice. This response is not intended to be anything other than my unresearched viewpoint, based on the very limited facts available to me. Any variances to the fact pattern(even slight) and/or additional information could effect the response and/or outcome of your matter. Please also note that it is possible that the results under your fact situation might possibly vary from jurisdiction to jurisdiction and that it is recommended that you consult a bankruptcy attorney in your jurisdiction. Furthermore please note that legal information is not legal advice and that I am only authorized to practice law in the State of Illinois.
I agree with the advice you have received, but you are able to amend far into the future. You may not have to amend if it is a chapter 7 case that had no assets for distribution. You must check the local case law with your attorney.